You can, but it will cost you dearly, and the tools needed to open an Air are not the easiest to come by.
You can get a pentalobe screwdriver from iFixit, then you need a T5 Torx for the screw holding the SSD in. Then of course you need one of the special form factor SSDs for the Air. OWC is the only place I've seen selling them, and once you wake up from the sticker shock, you'll see they only sell up to 480GB models. Which is probably close enough not to really matter, but whether it's worth almost $1100 is an entirely different question.