There are many reasons to turn a doorknob lock around. Placing the lock on the outside of the door is wise when the room is occupied by toddlers. This will prevent them from accidentally locking themselves into the room. If you are sharing a home with roommates, having the ability to lock your bedroom door while you're away ensures that your possessions are secure. On the other hand, accessing the lock from inside the room gives you control of your privacy, and keeps out unwanted visitors.
Remove the two screws from the doorknob cover plate on the lock side of the door, using the Phillips head screw driver.
Gently pull both knobs away from the door, being careful not to spill any parts of the floor. Leave the striker (the cylindrical metal piece that extends from your door's edge to the center of the doorknob hole in the door) in place.
Slide the doorknob with the lock on it back into the hole, on the side of the door where you wish to have the lock. Line up the screw holes on the knob with the screw holes on the striker.
Slide the other knob back into the hole on the opposite side of the door, again lining up the screw holes of the knob with those of the striker.
Reinsert the two long screws, tightening one partially, then the other partially, then the first one the rest of the way, followed by the other.
When working on an exterior door, never place the side of the knob with the screw heads on the outside. Anyone with a Phillips head screw driver can simply remove the screws and open the door.