This is explains how to change the factory internal battery in the Yamaha QY100 sequencer.
What you need:
CR 2032 Lithium ($2)
Philips head screwdriver
1. Unscrew every screw in the back. A gold colored screw is at the position shown.
2. Pull back the casing slowly to reveal the copper shield. Mind the wires in the bottom right.
3. Unscrew the two copper colored screws which hold the shield down. Open the shield forward, still connected.
4. Push the old battery further into the case from the front (left to right in the pic). Remove the battery. You can also gently use a small screwdriver to pry it loose. Replace with a fresh battery.
5. Carefully re-assemble. Power up. The screen should say "Factory Preset" on its first startup.