If its got rear discs mark it shud automaticly adjust itself via the calipers,if its does still need adjustment tho i wud check 1st to see if the arms are fully returning on the caliper,if there not it wont adjust up itself...adjusting the cables will just pull the arms on further and cause bind....
If it has drums i wud slacken of the cable,the remove the rear drums and manually adjust the automatic adjusters(as they dont work well( until you can get the drum back on with just a bit of resitance,the adjust the cable as requiered.