The battery in your generator will have a bracket holding it in place. You should see two rods that have a hook on the end. The other end will have a nut. Loosen the nuts on the rods. They might have to come all the way off to get the hold down out of the way. Before removing the battery, make note of the position of the terminals. Place the new battery in the generator and retighten the nuts for the hold down.
Make sure to connect the red wire to the positive post and the black wire to the negative post on the battery.
If you reverse the wires, it could ruin the generator. If you need further assistance, please submit more details.
Here is a link to order any parts you need.
Sears Parts Direct
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services