How do you replace lock cylinder mustang?

If you are talking about the ignition lock cylinder, turn the key to the accessory position, insert a paper clip into the little hole on the face of the lock cylinder to release the lock pin, turn the key a little further in a counter clock-wise direction and the lock cylinder will pull out.

If you are talking about a door lock cylinder, remove the door panel, roll the window all the way up, pop the little clip loose on the lock rod that is hooked to the back of the cylinder and remove the rod from the lock arm, then pull the 'C' clip out that is holding the lock cylinder in place and push the lock cylinder out.