You can just delete .git. Typically:
rm -rf .git
Then, recreate as the right user.
In windows, type rmdir .git or rmdir /s .git if the .git folder has subfolders.
If your git shell isn’t setup with proper administrative rights (i.e. it denies you when you try to rmdir), you can open a command prompt (possibly as administrator–hit the windows key, type ‘cmd’, right click ‘command prompt’ and select ‘run as administrator) and try the same commands.
rd is an alternative form of the rmdir command. http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/rmdir.mspx?mfr=true