These lenses are a little tricky, but this should help:

  1. Hold on to the lens securely, and with the other hand pull the side of the frame up and away where the frame inserts into the lens slot. Repeat with the other side.
  2. Then remove the nose bridge. Pop that off from the bottom first, then the top will release.
  3. Now with the new lens, work in reverse. Start with the nose bridge; set the top in first and then push it into the bottom, popping it into place. Then push the frame into the slots on the sides of the lens. 
  4. Do a little flex test to make sure everything is secure, and you're good to go.