Just exchanged the stock 2TB drive for a Seagate Archive 6TB drive. Ordered the 6TB from Amazon as a 'Backup Plus Desktop Drive'. Was expecting a 'Desktop version of the HD (based on online teardowns of the 5TB version), but found the Archive drive instead.
A few notes:
Didn't have a spudger handy, so used a couple of sturdy x-acto knives to pry the base off both units. The Airport was the trickier one, and I broke one of the plastic retaining tabs in the process. However it fits back together just fine (there are 8 retaining tabs in total I believe).
Removing the 3 tiny connectors from the circuit board is fiddly, so be gentle. Others have damaged theirs. The largest connector has side clips that need to be squeezed (I used small pliers) before lifting up towards you. The 2 tiny ones I very gently pried up under the cable with a small screwdriver. This lifts the cable end of the plug up, and gently levers the connector out. Returning the small connectors was simple - just press them straight down into their socket with your finger.
I removed the ribbon cable to the drive at the drive end (NOT at the circuit board). Gently pry the socket connector up from it's detent in the hard drive. It then can be pulled gently to the side, leaving the one end connected still to the circuit board.
Last cable to move is the mains power socket. It simply lifts out of a slot in the outer case.
Cutting the rubber 'shock bumpers' to fit the new drive was no problem (it has no machined lugs on the base, but the overall drive is the same size). Used x-acto knife and carefully cut where required. However, the bumpers become very thin in that corner, and may not provide quite as much protection after cutting.
Reassembly was very simple, with 2 notes:
The rubber base bumper (between the drive and retaining plate) must be pushed quite tight into the ends of the case. Otherwise there's insufficient clearance for the plastic outside base to reinstall. I used a flat screwdriver to press the rubber down in the corners. It ends up looking slightly 'curved' (further into the case at both ends) when finished.
The small retaining plate the holds the drive (and crossing cables) in place is quite a tight fit. I had to press hard on it to get the 4 short retaining screws started. The drive was properly seated, so perhaps the new drive is fractionally longer?
Overall, I'd say this was a very easy upgrade. I'm not a computer guy at all, but very comfortable with small hand tools, if that gives you any indication of the minor skill level required. Patience and care are the key to this job!