What are steps to replace windows 7 with windows 10 on Parallels 10.2.1

Hello

I am on Mac Yosemite 10.10.5. I have 4GB RAM.
I have Parallels Desktop 10.2.1, I installed on trial about a year ago to test it out.I installed Windows 7.
In my testing it appeared to need more ram than i have. It did work nicely if I did not use any mac osx (e.g. closed firefox, mail and pathfinder (finder alternative)). So it worked for me as an either/or.

Now I wish to trial again but this time the app I am thinking of buying needs Windows 10.
Do I need to reinstall Parallels to run Windows 10? (I did not know if a parallels version is linked to a windows version)
To replace windows 7 with windwos 10, do I just delete the directory or is there a Parallels wizard? I have no idea.

I would only need parallels for this new app. App is only available in windows.

I hope this makes sense
regards, and thankyou for your help.