Try this; it worked for me:
In your VM, go to https://www.microsoft.com/en-us/software-download/windows10ISO
Download and run the Media Creation Tool
It should bring up a window with the choice to upgrade your PC
Click this option, and it should upgrade your VM to Windows 10.
Of course, YMMV, and it may not work at all, but it upgraded to Windows 10 for me without a hitch from my Windows 7x64 VM.