Just like bigrock said. Pull the top & bottom of the frame apart to unsnap the lens. Take the new lens and align it in the frame groove. There are two tabs on the top and three on the bottom of the lens. First align the top tabs. Take a look inside the frame groove and see where the lens tab snaps into. This will give you an idea of which way to align the tabs. Once roughly aligned, firmly press into place.
After the top two tabs are in place, do the same for the bottom three. First align the then firmly press into place.
Sometimes lenses take some firm pressure to align and snap in. Don't worry the lenses are very strong, just don't bent them and you'll have no issues. Once you change the lens once, you'll be able to do it in a minute easily.