There's very little you can do to upgrade most modern Mac laptops. Even when parts aren't soldered directly to the motherboard (as is the case for RAM in all Retina MacBook Pros and MacBook Airs), Apple has a penchant for proprietary connectors that can make it hard to find aftermarket parts.

Other World Computing has built its business around making and selling upgrades compatible with Macs, and now it's finally selling aftermarket SSD upgrades that are compatible with MacBook Airs and Pros manufactured in or after 2013. That was the year Apple began shipping speedy PCI Express SSDs in its Macs, but they all used proprietary connectors instead of standard (but then either nonexistent or uncommon) interfaces like M.2. OWC's Aura drives come in 480GB and 1TB capacities, and they'll work in all modern MacBook Airs and Retina MacBook Pros (you can check the site for specific model compatibility).

OWC says the drives come with a three-year warranty and will begin shipping in late March. It also says that "you’ll never need to compromise your data by relying on complicated software hacks or TRIM enablers to get the most from your upgrade," which either means that the drives natively support OS X's TRIM feature somehow (not normally the case for aftermarket SSDs) or that the drive controllers' normal garbage collection capabilities work fine without TRIM.

The main downside is that the drives are pretty costly compared to standard 2.5-inch SATA SSDs, though they aren't too far off from the price of standardized M.2 SSDs for regular PCs. The "introductory price" for the 480GB drive is $347.99 for a bare drive or $399 for an "upgrade kit" that includes a USB enclosure for your old drive, while the 1TB drive and kit run $597.99 and $649 respectively. After the introductory offer is over, the 480GB drive will usually sell for $379.99 and the 1TB drive will normally cost $649.99.