Just hold the frame firmly with your one hand and open the frames (open the arms if they are folded). Use the other hands to hold the broken lenses (be careful, don't let the broken lenses hurt your hands) And wiggle the lenses back and forth to make the lenses lose from the frames. Usually, you can remove the lenses in this way. If can't. you can observe to see if there are thumbtack on the frames. If so, you shall first loosen the thumbtack before you remove the lenses.
It is not recommended to replace half frame reading glasses by your own because it is more complicated than full framed glasses. They use a wire to firm the lenses and frames and it is not an easy thing just like opening the frame and allowing the lens to fall out. Take it to the optical shop where you purchased the glasses and request their assistance. They may do it for free.
Usually, half framed glasses come with thumbtacks. So, to remove lenses from frames, you need first to loosen those thumbtacks. Once that is done, you can hold on to the frame with one hand and take the lens with another and wiggle lightly back and forth the lenses. In such a way you can have the lenses removed and if not, you should probably seek professional help. Take it to the shop where you bought it and have it handled by the staff there.
I suggest you can hold the frame firmly with one hand and remove the lenses with another hand carefully. Remember to wrap a soft cloth around the lenses to avoid scratching and do not push too hard. I think you can also go to some formal glasses stores and ask the technicians there to help you with the problem. Hope this will be helpful.