The belt on lower deck has about 3 - 4 inches slack. How do I tighten or is belt wore out?

Are you certain it is routed correctly ? See if you have a diagram either on the deck itself or possibly in your owners manual. If this is correct I would check that all tensioning springs are in place. It is possible one has snapped or vibrated lose, again I would check you manual or perhaps company website for correct positioning. I have not seen a belt stretch 3-4 inches without breaking.

Good Luck,
Morrowrj