To remove the lens from the frame, locate the small locking tab on the inside of the frame, near the temple. While holding the frame, push in the locking tab and pull the lens down and out.
To install the lens, insert the alignment tab on the bridge side of the lens into the groove near the bridge of the frame. Then connect the lower alignment tab on the same side of the lens.
Once those two tabs are seated into the frame, push the other side of the lens upward into the locking tab until it clicks into place.