If you are talking about the belt on the mower deck, I am including a link to the ownersâ€™ manual for this model, please click here;
. The section on belt replacement starts on page 24; there are also illustrations of the belt and pulley positions on this page.
If you are talking about the ground drive belt, lower the mower deck all the way down. Remove the mower deck belt. Remove (cut) the drive belt if it's still there, and get it out of the way. Take the new belt and put it around the pulley on the engine pulley. The engine pulley is a stack pulley. The drive belt will go on the top (little) one. Move the belt above that little pulley. Then pull the belt back to the transmission pulley and go ahead and feed it around that pulley making sure you get it in the groove of that pulley all the way in the back. Then on the left side of the tractor you will notice a belt keeper that kind of makes a box on the end of it. The belt will need to be put inside that box (it has an open end to it, so just push it in there). Now lock the brakes down on the tractor. The belt should still be above the engine pulley, go ahead and put it on that pulley (the little one). Next take the belt and push it onto the V-idler and the flat idler that are on the right side of the machine close to the engine pulley. Once you get it there, you're done. Release the brake and make sure you have tension on the belt and check to make sure it is under any belt keepers and not "on" them.
Once that is all done, put the deck belt back on the engine pulley. Raise the deck back to whatever height you cut at and you're now ready to mow.
If you require further assistance please resubmit your question with more details. I will be looking forward to helping you again in the near future.