Apparently an SR626SW or equivalent (https://casiofanmag.com/batteries/module-5125)
Watch out for the little spring that might be connecting the module to the back panel. If there is one it will try to escape.
Water resistance may be compromised afterward- I think the gasket is supposed to be replaced with the battery.
Dave
Thanks for that Dave I will give it a go later.
A little smear of Vaseline on the O-ring will help it to fully seat back into the groove, and not be nipped by the case back.
Those octagonal O-rings will struggle to go back in otherwise.
My 1990's GW6900 is still using the original case back seal, and is good enough for swimming.
Unfortunately the alarm went AWOL when the spring disappeared (or became detached from the circuit board)