Open the lock with the existing combination. Inside the flap, you'll see a small chrome lever. Push the lever to the left (as you're looking into the box) and **** it up into the offset slot. This puts the lock into the "learn" mode. Select your new combination and rotate each wheel until the numbers line up at the opening index. Flip the lever out of the "learn" position, and back into the "home" position.
Test your new combination several times, before closing the door.
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Open the lock with the existing combination. Inside the flap, you'll see a small chrome lever. Push the lever to the left (as you're looking into the box) and **** it up into the offset slot. This puts the lock into the "learn" mode. Select your new combination and rotate each wheel until the numbers line up at the opening index. Flip the lever out of the "learn" position, and back into the "home" position.
Test your new combination several times, before closing the door.