Why would you have windows 10 if you don't want it? Anyway what did you do to install windows you can see if that software has an option to remove windows as well
I'd say, OP wants the space occupied by Windows to be available for Android.
@ron8143: Make a test. Boot into Android, install a file manager, and see whether you can access Windows files. If you can, you could reclaim this space by just deleting (most of) Windows.