I would imagine you would have to carefully remove the bottom cover to expose the internal cards and look for the battery holder. I've done this with my Korg Triton Pro when I had a very loose Main Output Jack. I actually resoldered the loose solder connections on the circuit board and it worked like a charm. I've never replaced a battery, though. Should be easy enough if you are careful, confident, and handy.

Otherwise, bring it into a repair center.

Al