Let's begin shall we.

We start with the Central Locking Remote

Start by remove the first cover to access the battery. This is best removed from the key ring corner with a flat bladed screw driver to pry up and off.

There is a cover that hides the battery still (which isn't in the second photo, my bad), this is also removed with a flat bladed screw driver.

Remove battery from Fob (hey look, internal battery cover came back)