Remove the battery by unscrewing the top and popping the green battery out of the case. You can unsnap the battery holder from the cap if that makes it easier, although it is not necessary.
Note: the white battery is a dummy battery and should never be replaced.
Put the new battery in, re-connect the battery holder to the top if you removed it, and re-screw the cap on Alice. To screw the cap on, position the light slightly to the right of the arrow on the base of the gauge, replace the cap, and twist slightly clockwise.