You can take your frame to any optical store to get new lenses put in. The optician will do it for you. But make sure the new lenses fit your frame properly. If they can't fit your frame, you should get a new pair of flexon glasses.
Of course you can. That's what I usually do to save money. Many optical stores have policies regarding using the patient's old frames. You can go to your nearby optical store and and see who can make lenses for your frame.