Remove the four bolts on the generator's front cover with a socket and ratchet. Pull the cover off the unit.
Loosen the negative battery terminal with a socket and ratchet. Remove the cable from the negative post. Repeat this for the positive cable.
Unthread the retaining strap over the battery. Lift the battery out of the tray and install the new one in the same alignment as the one you removed.
Replace the retaining strap, threading it through the clip on the inside edge. Attach the positive battery cable and tighten it with a socket and ratchet. Repeat this for the negative battery cable.
Position the generator cover back in place. Secure the cover with the four bolts and tighten them with a socket and ratchet.