Turn the pedal upside down.
Unscrew the bottom plate, which is held in place with a Phillips head screw, and remove it to expose the battery compartment.
Disconnect the old battery from the power connector. Discard or recycle the old battery according to local regulations.
Install the new battery, making sure that both snap fittings on the pedal's power connector are firmly attached to the battery terminals. Reattach the bottom plate.
Test the pedal. Plug a guitar cable into the pedal's input jack, and plug the opposite end into your guitar's output jack. Plug a second guitar cable into the pedal's output jack, and plug the other end into your amplifier's input jack. Turn the amplifier on, and step on the pedal while playing to ensure that it produces the desired effect.