I am working on a neighbors huskee slt4600h lawn tractor that wont start. I replaced the battery and it started once. When I tried to restart it all I get is a click from under the seat when the key is turned.

The click you are hearing under the seat is the solenoid. Most likely it is bad. Luckily it is a very easy and cheap fix. You can always use the "shade tree" test to make sure. Turn the key on, lock the parking brake with the unit out of gear and use a screwdriver to jump from oe pole on the solenoid to the other. If it starts, the solenoid is bad. The click you are hearing under the seat is the solenoid. Most likely it is bad. Luckily it is a very easy and cheap fix. You can always use the "shade tree" test to make sure. Turn the key on, lock the parking brake with the unit out of gear and use a screwdriver to jump from oe pole on the solenoid to the other. If it starts, the solenoid is bad.