I have a 11721, which although not identical, I’ll bet it works the same way. Place the front roller against a firm surface (I use my work bench) and press forward firmly with the handle. This compresses a spring in the belt tension assembly. Rock the body slightly to one side (I forget if it is to the right or left) and it will lock in the compressed position which gives slack in the belt. Slip the old belt off and the new one on, being sure to note the proper rotation of the new belt. Release the roller in the reverse manner.