The basics of how to change the battery are simple: a) prize the back off using a flat blade such as an old kitchen knife, b) take out the battery and replace it with one of the same size.
But you know what? It ain't really worth it. My wife's watch stopped while we were in holiday in Egypt. I scratched up the back using a knife I 'borrowed' from a restaurant, then discovered the battery I'd bought was the wrong size!
Seriously, I wish I'd just have waited and taken it into a jewellers.
Simon Croft, June 2010
