Turn the mower's power switch to the "Off" position.
Pull off the two plastic battery terminal covers.
Remove the black wire attached to the battery marked with a "-" by grasping the hex bolt firmly with an 8-inch adjustable wrench. Turn the wrench in a counterclockwise motion, letting go and grasping the bolt again when the wrench hits the battery. Continue this motion until the bolt and nut can be removed by hand. When the black wire is removed from the negative terminal, remove the red wire from the positive terminal using the same method. The positive terminal is marked with a "+."
Flip the hold-down rod to one side of the battery. The hold-down rod is a metal swing arm that ensures the battery does not move. Pull the battery out of the compartment.
Place a new battery in the battery compartment.
Attach the red wire to the positive battery terminal. Secure the wire in place using the hex bolt that held the original wire in place. Twist the bolt with an 8-inch adjustable wrench in a steady, clockwise motion until it cannot turn further. Cover the positive terminal and wire with the red plastic cover attached to the battery.
Affix the black wire to the negative battery terminal. Secure it in the same manner you used for the red wire. Cover the negative terminal with the black plastic cover attached to the battery.