Refer to the Schwinn bike-lock's user manual to determine the default lock-combination. Turn the tumblers on the Schwinn bike-lock so the numbers line up to that combination. Pull open the lock.
Push the small, reset lever found on the head of the Schwinn lock. Press it flat against the lock.
Set the lock combination by turning the four tumblers to the numbered combination of your choice.
Press down the reset lever on the head of the Schwinn lock. It should be flush against the lock.
Close the lock and scramble the tumblers. Position them to the new combination. Open the lock to verify that it works.