To change the code on a Presto lock, enter the correct code and open the case. Now push the button you use to open the case to the open position and hold it there, now push in the clamp and push the opener more and it will lock in place. Enter the new code and push the clamp part again to set it. Do to other side now.
All the best. Please be sure to hit accept as that is the only way I receive credit for my response to you today.