Very easy. 5-10min of work start to finish really
Just need a soldering iron, something to pry on the battery as you heat the solder, and a replacement tabbed battery for the board. Heat, pry, lay down new, re-solder down, done. I do this somewhat often on Gameboy stuff which is smaller but all the same.
Best deal I know online is starwander/mortoffgames up on ebay. Has all the different versions of batteries that Nintendo consoles and Gameboy's have gone through over the years with tabs on them.