I want to install a guest OS from scratch. How can I remove the existing VM? Do I need to reinstall Parallels Desktop for Mac?
There is no need to reinstall Parallels Desktop. You can completely remove any existing VM by deleting its files. By default, they are stored at Mac HD - Users - <your user account folder> - Documents - Parallels folder. Then install any guest OS in a new virtual machine.

I want to change my virtual machine settings in Configuration Editor, but the guest OS immediately launches when I start my Parallels virtual machine.
Press and hold the Option key on your keyboard and double-click the VM icon (the “.pvs” file. Hold the Command key for 20-30 seconds until you see Configuration Editor.

How can I connect an ISO image of a CD-ROM to the VM?
To use a CD-ROM image file in Parallels Desktop please follow the steps below:

1. If the VM is stopped, click the CD/DVD-ROM link in the main Parallels Desktop window, choose the Use image file option and specify the path to the file using the Browse ("...") button below. Then click OK to close Configuration Editor and click the Save button to save the changes you've made to the VM.
2. If the VM is running, choose CD/DVD-ROM from the Devices menu in the Mac menu bar, select CD/DVD-ROM, then choose the Connect Image option and locate the disc image file.

How can I use floppy disks in VM?

Parallels Desktop for Mac supports external USB floppy drives in your guest OS. Just plug the USB drive into your Mac computer, connect it to the virtual machine using the Devices menu on the toolbar, and it will appear in the guest OS as a new USB device.

If your guest OS doesn't support such a USB floppy drive, but you still need to boot the VM using a floppy, you have to create an image file of your floppy disk using the Disk Utility on your Mac and connect that image to the VM’s virtual floppy drive. Here’s how you do it:

1. In the Finder open Macintosh HD - Applications - Utilities - Disk Utility
2. Insert the source diskette (the image of which you want to create) into the external USB floppy 3.5 disk drive and connect the device to the idle USB port of your computer.
3. Select this floppy from the Disk Utility device list.
4. Click the New Image button on the top of the Disk Utility window. Specify the name and location for the image, then set Image Format to read / write, and Encryption to none.
5. Click the Save button and wait until the image is created.
6. Locate the new image, right-click (or Ctrl+click) it, and choose Get Info from the pop-up menu.
   Expand the Name & Extension sectioin, change the file's extension from .dmg to .fdd, press ENTER, and then click the Use .fdd button in the pop-up window.
7. Launch Parallels Desktop, click the Floppy link in the main window to open Configuration Editor. Select the Connect at startup option and locate the floppy disk image file using the "..." button. Click Save to save changes and then OK to close Configuration Editor. You can also connect the floppy disk image file when the VM is running using the Devices menu in the Mac menu bar.

How can I change Right Enter (Numpad Enter) key behavior (AltGr or Enter) in VM?
If you want the right Enter key to operate as the Enter key in your VM:

1. Stop all the VMs, close all the instances of Parallels Desktop.
2. Open Macintosh HD - Applications – Utilities – Terminal.
3. Type the following commands in Terminal (be ready to enter the Administrator password when prompted):
   • sudo touch /Library/Parallels/.keyboard_config
   • sudo chmod 666 /Library/Parallels/.keyboard_config
   • sudo echo "nonumenterisaltgr">/Library/Parallels/.keyboard_config
4. Close the Terminal.
5. Launch Parallels Desktop and check the result: the right Enter key should function as the Enter key in your VM.

If you want the right Enter key to function as the AltGr key in your VM:

1. Stop all the VMs, close all the instances of Parallels Desktop.
2. Open Macintosh HD - Applications – Utilities – Terminal.
3. Type the following commands in Terminal (be ready to enter the Administrator password when prompted):
   • sudo touch /Library/Parallels/.keyboard_config
   • sudo chmod 666 /Library/Parallels/.keyboard_config
   • sudo echo "numenterisaltgr" > /Library/Parallels/.keyboard_config
4. Close the Terminal.
5. Launch Parallels Desktop and check how the right Enter key functions.

I already have Windows installed in the VM. How can I make more space available on the VM hard disk without losing data on it?
You can expand your virtual hard disk and create a new partition on the added space. Please follow these steps:

1. Open Macintosh HD - Applications - Parallels - ImageTool.
2. After Parallels Image Tool starts, click Next.
3. Select Hard Disk as the device type.
4. Select the Increase the size of the Virtual Hard Disk option and click Next.
5. Click Browse to select the virtual disk you want to expand.
6. The path is defined in the Resources section of Configuration Editor.
7. Specify the desired size in the New Size box. Click Next.
8. Now, that the Image Tool is ready to perform the required operation, please verify the settings and click Start.
9. This operation requires approximately 5 minutes. (The time depends on the amount of data already stored on your virtual hard disk).
10. After the operation is complete, click the Exit button.
11. Start your guest OS (Windows).
12. Choose Run from the Start menu. Type diskmgmt.msc in the input field and press Return to open the Disk Management utility.
13. Right-click (or Ctrl+Shift-click) the Unallocated Capacity bar and select the New Partition option. After that, click Next in the dialog box to continue.
14. Select Primary Partition option and click Next.
15. Specify the size of a new partition and click Next.
16. Specify the drive letter and click Next.
17. Select Format partition using the NTFS file system. Specify the volume name and click Next.
18. Click Finish to start formatting.
19. A New Volume will appear in the Disk Management window and in My Computer folder.

I want to share a VM with several user accounts on my Mac. Is it possible?
Yes, it is possible. To share a VM between several user accounts do the following:

1. Move the VM files (.pvs and .hdd) to your shared folder (for example, Macintosh HD - Users - Shared - VM folder).
2. In the Finder, right-click (or Ctrl-click) the folder with the VM and choose Get Info from the pop-up menu.
3. In the next window expand the Ownership&Permissions group.
4. Expand the Details group.
5. Set the access level for Others to Read&Write.
6. Click the Apply to the enclosed items button.
7. After it's done, each user of your Mac should be able to access these VMs.