1.Turn Off the Cub Cadet Lawn Mower
2.Remove/Disconnect the Spark Plug from the lawn mower engine

3.Check if the belt has any tensioner or if some adjustment on the mower is keeping the belt tight?
If yes,release the tension on the belt.

4.Start working the belt so that part of it is worked over the edge of the pulley and slowly turn the pulley simultaneously,so the belt will come off the pulley.

Check the part(belt) and size number and make sure the new belt is same as the old belt and is suitable for Cub Cadet Lawn Mower.
You can also physically compare the old and new belts…the old belt should be slightly larger than the new one.

5.Reverse Step 4 shown above…so that the belt slowly slides over the pulley and into its place

If the new belt slips on the Cub Cadet Lawn mower easily,make slight adjustment to the tension to tighten up the belt.