12Pack CR2032 Battery, CR2032 3V Button Cell Lithium Game Batteries with Solder Tabs for NES, SNES, N64, Gamecube, Super Mario RPG Game Battery Replacement

You need a 3.8mm Gamebit to open it. Batteries are typically soldered to the board, so you would need to remove and replace it. They sell CR2032 batteries with the tabs that are needed online.