You have to take the door panel off, obviously. You can then see the lock rod going from the lock stem inside, down to the key lock. This may have bent or need adjustment. If you have to replace the actual key lock, it is simply held in place by a C clip. Take some pliers and remove the C. Slide it out. The key part will then fall from the outside of the door.
My 79 does that,,,, i have to jiggle the key if i try to unlock w/ the key. I know it needs to be adjusted. If i have to replace the actual lock cylinder, i would need a new key, which sucks.
SgtDel