For this you will need:
The key in question looks like this:
The black part of the key is two plastic halves clipped together. There’s a little indent on the end:
Use something hard to pry the two halves apart in that indent (we are using a Victorinox 53201):
Inside you should find this:
The clear plastic piece on the right pulls out:
Reverse side:
The black plastic clips over the clear:
Pry those (gently!) apart and the clear will lift out of the black piece:
Inside you will find what is commonly called a watch battery – this one takes a CR2016 sized 3v coin battery (eBay US, eBay AU, Amazon).
The battery should remove easily:
Install the new battery:
Clip the clear plastic back into the black:
Install it back into the key:
Clip the two halves back together:
…and now test the key – you should be done!