Step 1: Even though Parallels Desktop automatically detects whether 32-bit or 64-bit Windows version is installed in the virtual machine, you can check which one you’ve got by clicking on Start menu > right clicking on Computer > Properties.
Step 2: Choose one of the links below depending on Windows version shown in computer’s properties.
Step 3: If you have Parallels Desktop Sharing Tools enabled, you can run the downloaded file from the Mac’s Downloads folder.
Step 4: Run MediaCreationTool.exe and follow the on-screen instructions to upgrade Windows.
Step 5: Once the upgrade has finished you can shut Windows down and rename it.