To remove the lens pull the upper part of the frame up, unlocking it from the two anchor points, then, we can remove the anchor point on the side and on the nose simply pulling out.
To mount the new lens we start from the nose inserting the lens in the slot, then we insert the lens in (under) the two metallic parts on the side pushing until the lens is in th same position as the frame in the lateral parts, you can start from a whatever one of the sides.
Finally, lock the two upper anchor points of the inner lens in the housing of the frame, making sure that the joins enter completely in the slot. Do not push too hard! If you see that the joins aren' t completely in do not push too hard, you risk to damage completely the goggle, but repeat the procedure until you find the right slot. The procedure is really quick, but you need to pay attentiont in the last part of the changing while inserting the lens in the joins.