First thing to do is open your Riddlebox with the given riddle answer. (The answer can not be changed unless the puzzle is open). Once you have the puzzle open you will used the screwdriver to unscrew the 2 screws along the collar. As seen below

After you've taken off the collar (be careful not to lose the very small screws) remove each letter ring. Each letter ring is attached to the actual cylinder lock mechanism. You will see that the letter corresponds to a slot in the metal. 

Simply remove each letter ring from its base and line up the letter you would like. Replace the rings onto the cryptex in the correct order, line up and replace the collar, and re-attach the two screws and your puzzle is set!