Take the fob off the keyring then slide the top part off like this.......
The two lugs securing each half of the fob which are at each side now exposed, can gently be released, allowing the two halves to be separated and access to the battery, a CR2032.
Taking note of the way the battery is fitted, remove it, then refit the new one in it's place.
Reassembly is a reversal of disassembly.
Brian
