Let's begin shall we.

We start with the Central Locking Remote

Don't immediately start trying to seperate the Fob shell, there is a hidden screw behind the Toyota logo

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.

The internal battery cover is a water tight seal and isn't clipped in. It can be removed in a twist motion while levering up.

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

Remember to keep the battery, or even replace with a new one. The battery covers can be used in the replacement shell as well.