Q: I plan on changing the battery on my 1-slot Neo Geo MVS (MV1FZ). Where is it located and what sort of battery will I need?
A: Below circled in red you will find the battery's location on the board. You can gain access to the board's location by removing the 4 screws holding the MV1FZ's black shroud on. The ideal replacement battery is a Panasonic 3V VL2020 Lithium Vanadium Pentoxide Rechargeable Coin Cell Battery, PCB Pin Terminal, 20mAh.
Again, though the battery itself is not pictured, the area circled in red is where it resides. To remove it you will have to flip the board over and remove the protective foam pad (if present) and desolder the battery from the board at both of its points. Installation is the opposite of removal.