If, like me, you have a Rotary Elite rectangular dual faced watch here's a link that shows how to replace the two batteries in it.

https://www.youtube.com/watch?v=AFtDTm3Z2YI&t=99s

Takes two batteries, Renata 321 for the black face and Renata 362 for the white. It also helps if you have a movement holder when changing the battery on the white face, though he doesn't mention that in the vid.

Saves paying a lot of money to have it done - cost me £40 last time, (and one AD wanted £70!).