Words of wisdom: Be careful that the screwdriver doesn't pop off and scratch your lens! Then you are really "screwed." It is probably a good idea to work over a table so that parts are not damaged or lost on the floor. Also make sure your hands are clean so you don't scratch the lens with dirt/residue from your fingers. 

First pop off the side shields so they are out of the way. Using a small flathead screwdriver, loosen the frame screw. This is the larger of the two screws (closest screw to the lens, has grooves). You may need to also loosen the temple screw slightly so the frame opens up enough to pop the lens free.

Once you have the lens out, place the new lens in and tighten up those screws in reverse order (temple screw, then frame screw). Make sure the lens is seated properly and is not shifting at all.

Repeat with the other side. Replace the side shields and you're good to go.