I have a couple of these masks, one clear and one black....I prefer the black one because it does not show the dirt that builds up like the clear one does. I have taken the clear one apart to clean.
It has been a couple of years since I did this. There is a clip behind the area that sits above the nose that retains the silicone skirt in place. If you gently peel the part of the skirt, at the top center of the mask, away from the frame, you will see the top of the clip...I believe it needs to be slid upwards (towards the frame of the mask) to release...a small flat-head scredriver is what I recall using to slide the clip up.
The process, obviously, will need to be reversed to secure the skirt when you put it all back together.
-Z