Depends on the model.

If it is one with no frame at the bottom, to change the right lens --

Hold the glasses in your left hand, temples pointing at the floor, your palm over the front, and your left thumb on the top of the frame going over the top of the right lens.

Put your right thumb next to your left thumb and then apply gentle pressure on the temple so that you can slight bend the top of the frame in the opposite direction of the lens. Be very careful so you don't bend the hinge.

There is a little tab that will come out of the corner of the frame by the hinge and then you can pull the lens out the rest of the way.