Once you have opened the watch case you will need to remove the old battery and replace it. There are a number of different ways in which batteries are held in
place the watch movement. Sometimes they are held in by a spring clip, sometimes by a retaining bar, sometimes by a plastic cover. In some cases
it will be neccessary to loosen or remove some screws to remove the retaining clip, in others the battery can be eased out of the battery compartment.
Here are some examples of the types of battery retainers you are likely to see:
Once you have removed the old battery, putting in the new battery is simply a reversal of the procedure above. Using the tweezers or other non-metalic tool ease the battery
back into place and, if required, screw the retaining bar back in place.