Chuck,
I would suggest you have the store from where the mask and lenses were purchased from do it for you. I'm sure you would be regretful should you accidentally break the mask in the process.
Having said that it is easy to do, one needs to be careful though. The lenses come with a little plastic tool, looks like a wedge. If you look carefully at the front of the mask around the perimeter you will see where there are little indentations or clips holding the inner portion of the mask frame and retaining ring together. Using the tool you wedge it between the frame and retaining ring at these points prying outward. This will one by one pop out a section of the retaining ring. Careful though as the retaining ring is the fragile piece. Once the retaining ring is completely removed, the lenses will drop out, put new ones in, retaining ring back into mask and make sure you have a good seal. You may have to work on the seal part, sometimes it can be a little challenging.