Over time your NES game batteries wear out, thus taking away the ability to save your game position. With the right tools and just a few minutes of effort, you can replace your old worn out battery and defeat the evil Ganon in the Legend of Zelda once more (without a 8 hour game playing marathon)!
Using the 3.8mm Security Bit, unscrew the 3-5 screws from the back of the cartridge. Our 3.8mm bit is designed to fit in screw holes easily, but this still may require some pressure. Lift up the back side of the cartridge. Be careful not to lose the screws!
With the back open, you can simply pull up the PCB and remove it from the cartridge case
You should see the battery being held in place by a firm clamp of some sort. Take note of which side of the battery is facing up (positive or negative). This will be needed later on. Take a flathead screw-driver and pry the battery out, you do not want to destroy the holder. The reason it is hard to remove the battery is because Nintendo actually put holes in the battery itself. Weird.
Place the new NES game battery in the holder and take a piece of electrical tape and place it over the battery/holder to keep the battery in place. Make sure to insert the battery with the same side facing up as you found it.
Put your NES cart back together. Start up a new game, save, then turn your NES off for a bit. If your replaced the battery correctly, the game will save your data like it should.