We do not have any written instructions for replacing the drive belt of a treadmill. Please follow the steps below to aid you in the belt replacement.
Unplug the treadmill and remove the motor cover. The cover has three screws along the front and screws from the bottom.
Loosen the rear roller adjusting bolts.
Loosen the bolts that mount the motor to loosen the belt.
Remove the bolt in the front roller opposite of the pulley. Count how many turns it takes to remove it. When you are putting it back together, turn the bolt the same number to insure the same placement of the front roller.
Remove the roller and replace the belt.
Reassemble in the reverse order.
You will have to push down on the motor to tension the drive belt. When pushing down have someone else tighten the mounting bolts.
I have provided the instructions of how to adjust the walking belt.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services