Roll back to Mountain Lion without time machine backup I've installed OS X Maverick which was downloaded by someone else and was registered to other apple ID, that's why I can't install maverick free apps like keynotes, pages...how can I roll back to my mountain lion or start a fresh install or change the registered user of maverick I'm using right now?
I don't have any time machine back up or any installation disk.
Open the Mac App Store.

Then go to the Menu "Store" > Sign Out.

Now you can login with your account and you should be able to download the desired software.