I need to know how to titan the gear belt I believe it's called. I know there's 2 belt, one is the drive belt which spins the blades. I need help with belt that's above that, which works with the clutch and gear box. I don't know what to a just to remove the slack?

Hi,
Most easily done with deck removed. Lock park brake and check the linkages. There is no way per se, except some of the flat idlers are bolted on a slot. Most are not, but you either have something crooked or a non-Deere belt.
Peace,
Clarke