In reply to Heike:
You have to carefully manipulate the lens from the frame. Have a look at this photo to see the cutout that fits into the frame part which may help. https://www.wizbiker.com/image/cache/catalog/Products/Endura/endura-shark-g...
Not same glasses, but similar type of fit and on mine I start by pulling the lens down in the middle until there is a small gap with the frame and then carefully bend to remove one end/side.
Putting the other lens back in is the next challenge! Easy once you know, but just be sure that it does slot properly into the side/end of the frame for safety. I’ve had mine come out where I thought it was properly held in.