To remove lens:
grip lens and pull out from the nose bridge end via the groove in the nose bridge. frame will need to be flexed a bit.
To fit lens:
push the outside lens notch into the arm end of frame and then slide the other side of the len into place via the groove in the nose bridge. Frame will flex to allow this. You may need to 'wiggle' the lens at the last stage to ensure it slides into the top of the frame.