When you press the "Start" button to begin charging the amp digits begin to flash. Rather than continue holding the Start button release it. The amps digits continue to flash. Press the "INC" or "DEC" button to change the current setting. Once set where you want it, press and hold the "Start" button again to begin charging.
You might want to save this setup, too, for later.

NOTE: The "Start" button might also be labeled "Enter".