Open the lock with the existing combination and pull up the shackle.
Turn the shackle to the 90 degree or 180 degree position and push it back down. You need to be able to push the shackle back in while leaving the lock open to hit the reset area.
Hold in the lock and put in your new combination. This combination will be the reset, so write it down and make sure it's something you can easily remember.
Release the shackle, rotate it back and close the lock. Scramble the pass code and then put in the one you changed. Open the lock with your new code to be sure it works.