@Hockeybod - if it's a late model Frog (which is very likely) then the procedure is essentially the same as the Jester i.e. open the case, swap the battery, close & secure the case.
It's a CR2032 same as in the Jester - there are only two screws to remove, as you look side on to the desk - at the rear/back end of the side panel remove the lower crosshead screw at each end & the bottom tray will be free to 'hinge'. Be careful when lifting the front panel - there is an earth cable between the processor and front panel that is shorter than the ribbon cables.
Points to remember...
The bottom tray locates in a groove in the case extrusion at it's lower/front edge - it can be a faff sometimes to locate it back in.
Put the battery in the right way round (yes - I know... it has happened!)
When done, power up the desk - go in to Super User & do a Reset/Clear all so that the internal patch tables reset - job done!