Some models of Master Locks can be reset. If one can be reset, the method to reset will depend on the model of lock.
A reset tool is needed for padlocks model numbers 175, 176, 177 and 178. With these models, the current combination must be known. "Set Your Own" locks have a preset combination depending on which model is owned. P-L-A-Y is the combination for model #643DWD.
To change the combination on any lock, the shackle must be opened when the reset process is started. After the combination is reset, one should write the number or word down in a secure place so it will not be forgotten.