How to install windows 10 with another version of windows 10 installed

[WiiUMasterGman]

I have tried to install windows again because I am trying to sell some old parts and prove it working. But I can't instal it on a partition with 200gb. It just says windows cannot be installed on this partition. I tried something before. Opening CMD and cleaning the disk but I don't want to do that now becuase I fear that would wipe the drive and then I would have to install windows all over again. What do I do?

 

[USAFRet]

Why would you need to install it again to prove functionality?

 

[WiiUMasterGman]

Why would you need to install it again to prove functionality?

Because I am using the same hard drive on a different computer. And the main reason why the person wants the computer is he or she wants to play WoW and I need to know if I can actually run it