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

  1. Remove the nose bridge. Pop that off from the bottom first, then the top will release. 
  2. The temple arms pop off by pulling up and away from the lower attachment, in an upward motion. So hold on to the lens securely, and with the other hand pull up and away where the frame inserts into the lens, first releasing the lower part from the slot and then whole thing should release. 
  3. Now with the new lens, work in reverse. Pop the lower part of the frame into the slot, and then make sure the top part seats securely. It should pop into place. Same thing with the nose bridge; set the top on first and then push it into the bottom, popping it into place. 
  4. Do a little flex test to make sure everything is secure, and you're good to go.