Now I am going to say this with the following warning:
If you do not feel confident in your abilities or you do not have the authority to dismantle your board than do not read on.
OK - if you unscrew the back panel on the board you will probably be able to see the battery. From my experience, they usually make them easy to find/replace.
If I get a chance, I will have a look on the web for the
manual and/or tech support. If they are available. I assume that you have already done this?
If not, do so. If nothing can be found,
send an email to the
Yamaha head office for the US/CAN. If there is an audio service centre close to you,
call them and ask for their advice. After all, this board is 8 years old and well out of warranty, so you do not have to go to an authorised service centre. Especially for something like changing a battery. Was it a more specific problem, maybe.
Good luck