Revert back to Win 7?

Not sure what happened...Operating Systems are really not my thing...

So a relative came over and the computer asked if they wanted to upgrade...he said yes...it now has Win 8 on it from win 7.

Apparently none of the files that were on it are not now, perhaps they're in a different partition or something...

Anyone have an idea of what might have happened and/or how to get Win 8 off and restore Win 7?

Thanks,