On the bottom of the hub, there are 4 sticky pad feet. Take those off and there will be a Phillips screw under each. Remove the screws and pop the top off. The antenna wires are soldered directly to the board so you'll need to use a soldering iron to heat up the connections to remove them.

Next, you'll need some U.FL connectors to solder to the board. Links to everything I used are below but you can get from cheaper sources with a bit of searching. Particularly the antennas. The main reason for me going with the supplier I chose for antennas, I don't really care what things cost. As long as it does what it says, I'm willing to spend more on better quality. I realize this isn't always the same approach others take. As with any mod, there's always some implied risk and you just mitigate what you can within reason.

The two larger pads are the ground pads and the one to the side where the center wire was soldered goes to the center pin of the connector. The U.FL traces/pads are already there they just opted to not solder on a connector and just wired the antenna straight to the board. Due to the size of the connector, I highly recommend a good set of fine-point tweezers and a dab of flux, even if you're using rosin core solder.

Drill some holes for the RP-SMA connectors in the lid, paying attention to where things line up on the board. A 6.5mm drill bit makes a perfect fit. If you use large antennas keep in mind their weight. Putting both on one side may cause the hub to tilt over which is why I went with one on the back and one on the side. If you use large antennas, be sure to tighten down the connector to the lid very firmly so the connector doesn't twist when you put the antennas on.

Carefully align the U.FL plug to the connector and push it strait down. You should feel it click into place. Screw everything back together and stick the feet pads back on if you haven't completely mauled them taking them off.

It took me longer to find what parts I wanted to buy than it did to do the mod. I went with a U.FL to RP-SMA pigtail and RP-SMA antennas so if the connector wears out from swapping antennas or whatever then it's the antenna that wears out and not the pigtail. If you have spare antennas laying around, you could save a little bit on the mod by using regular sma connectors.