Most locks are not meant to be taken apart nor have their individual parts replaced. Occasionally, you may need to replace the shackle -- the U-shaped bar used to secure your valuables -- because of damage or possibly because someone cut through it. You won't be able to replace the shackle on most locks simply because you would have to take the entire lock apart to do so. Some locks, however, are designed specifically with a replaceable shackle and need only a small Allen wrench to remove and replace the bar.
Unlock the lock with the appropriate key.
Locate the small Allen wrench hole near the bottom of the shackle.
Insert your Allen wrench into the hole at the base of the shackle and turn the screw inside counterclockwise to loosen it.
Pull the shackle straight out of the lock base.
Insert the replacement shackle into the lock base and tighten the screw to secure it in place.
This process works only on locks designed to allow for shackle replacement. If there is no shackle screw hole on your lock, this process will not work.