1. Unlock and move the carriages to the opposite sides of the rails.
2. Turn the SliderPLUS upside down with the wheels facing the ceiling.
3. Push the belt clip at the spot that the screw is at, as opposed to the part closer to the belt's end.
4. With some force, this should allow you to push the belt out and replace it with the other belt.
It is important to push closer to the inside of the belt clip, where the screw is, instead of the flatter part closer to where the belt comes out of. Kindly see the photo below of what this may look like.