The Citizen Navihawk differs from a lot of watches, in that it uses both an analogue and a digital display. It has the ability to display the time in 30 different cities around the world, and is also equipped with three alarms and a timer. Replacing the battery requires you to remove the watch back, which is secured with screws. This delicate operation should be performed in a dry, well-lit area.
Place the watch facedown, with the rear facing you.
Locate the six holes on the back of the watch. Line up the watch case removal tool, so that its pins match the holes on the back of the Navihawk. Insert the pins into the holes.
Turn the tool counter-clockwise, until the back cover separates from the watch. Remove the back cover completely.
Remove the old battery. The old battery is located under a diamond-shaped cover. Remove the cover with tweezers, and lift the old battery out of the housing.
Place the new battery in the housing. Replace the battery cover.
Press the "+" side of the battery twice. Press "AR" -- All Reset -- to reset the watch.
Replace the back cover and twist the watch case removal tool clockwise, to screw th holding screws back in. Tighten the back, as much as possible.
If you are not confident performing this operation, then it is best to take the watch to a watchmaker or watch store.
Performing this operation yourself will void any warranty from Citizen. For this reason it is a good idea to have the battery replaced by a Citizen approved facility.