1. Hold down the larger of the two buttons until you notice the digits in the lower left corner blinking.
2. They will say 12 or 24 hour, whichever you have it set or want it set to, use the smaller button on the right to toggle through your number choices.
3. Once you settle on the appropriate time in the slot that is blinking, press the larger button again and it will go to the next digit.
4. Repeat until all digits have been checked.

It is much more simple in practice than explanation.