Try a search for "repair 50mm focus" and you will find a good bit of information. This one looks like it might be pertinent:
Pentax 50mm repair 1 .
From personal experience:
Do your research up front, make sure you are comfortable doing this, assemble all the tools you will need, make sure you have lots of time, no caffeine, a well lit work area, magnifying glass or glasses, computer near by for web reference.
Get a large zip lock bag, some of the dis-assembly causes small springs and parts to fly about randomly, take it apart in the bag and at least the parts are confined to the bag.
Do not attempt if you are not willing to fail and dump the lens. This is always a possibility, especially the first time. It gets easier as you learn all the tricks, and hazards.
Good Luck!