Unlock the bike. The original bike combination set by the manufacturer will be written on a piece of paper included in the original packaging of the bike lock.
Push the reset lever up. According to Masterlock.com, the lever should be facing down after you unlock the lock and pull out the cable. Flipping it up allows you to change the combination.
Turn the bike lock dials to the new combination that you would like. When you have determined your new combination, flip the lever back to its original position.
Lock the bike by pushing the cable in and scrambling the bike lock combination.
Test the new combination to ensure that it is correct. If it does not work, try the original combination in case (perhaps the change did not take effect).