how do i tighten my mower deck belt?

Answer 1

Most mower belts have a tensioning pulley to keep the belt tight. Check under the deck and see if yours has "froze up" or something is preventing it from operating. The spring may be fouled with grass clippings or the spring rusted.

Answer 2

Make sure that the idler puley is moving freely and the spring attached to it is clean and not stretched. If those are ok, then you will need to replace the belt because it is likely stretched.