How do you change the belt on a model 201 Singer sewing machine?

Not all 201s use motor belts. Those made in the U.S. are directly driven through a gear system. If your 201 was made in the U.K., it will use a belt drive. In that case, you loosen the one bolt that attaches the motor bracket to the body of the machine, place the new belt around the balance wheel and the motor pulley, slide the motor bracket up or down so the belt is just tight enough to drive the machine without slipping, and tighten the bracket attaching screw.