To have a smooth upgrade with aborting, ensure:-
Ensure you have the most recent update of Parallels 10 or running Parallels 11
Your copy of Windows 7 or 8 has been activated on your system
Ensure you have aLlocated enough memory to the virtual machine. At least 2 - 4
GB
I allocated the max of 4 because I only run VM when I need it.
This will get you the instructions.
http://kb.parallels.com/en/122884
Have fun
Cheers f