Remove the safety key from your Weslo treadmill and unplug the electrical cord from the power outlet. Injury may occur if the treadmill starts while you are adjusting the belt.
Locate the roller adjustment bolts in the rear of the belt unit. Find the Allen wrench that came with your treadmill or find a regular Allen wrench that fits in the roller adjustment.
Insert the Allen wrench in the roller adjustment bolts. Rotate the bolt a quarter turn in a counterclockwise direction if you are trying to tighten the belt or in a clockwise direction if the belt needs to be loosened a bit.
Grab the treadmill belt with your hand and try to pull it away from the walking platform 2 to 3 inches. If you cannot pull the belt up this far, you need to loosen the belt because you have overtightened it. If you can lift it more than 3 inches, you may want to tighten the belt just a bit more to prevent slippage.
Plug the power cord back into the electrical outlet, insert the power key and power up the treadmill. Let it run by itself for a few minutes to make sure the belt is at an appropriate tightness level. Step onto the belt at a low speed to test it after a few minutes have passed. If the belt slips or seems loose, unplug the outlet and start the process again until you are satisfied with the tension of the belt.