It's a very easy process. There's two pieces of plastic surrounding the column, the top and bottom. There's 2 phillips screws going in from the bottom that hold the two together. Undo those screws, tilt the wheel up and remove the bottom one, it should go towards the dash and down. The top goes in the same way. Actually, I'm not sure which half comes out first but if you fiddle with it, you'll find out.
Once the plastics are off, there's just screws holding the multifunction switch in place. You'll see them, one at the top and one at the bottom. They go from the driver's side and screw into the passenger side, into the column. Once those two are out, the hardest part begins. A small pocket screwdriver is helpful, maybe 2 or 3. And an extra set of hands. The wiring connections are a bit fiddily. I find it easiest to get a screwdriver up underneath the tabs that 'lock' it in place and lift those while pulling the connection outward. It's tough because there's 4 or so of those tabs per connection. Start on one side, pull it out a bit then work on the other side. Just wiggle it back and fourth. Don't pry too hard, you'll break a tab.
Once you get it out, it goes pretty smooth. Push the connectors back in place and put everything back! One thing to note, go through each and every one of the wires and push them into the switch connector. A few of mine came out when I pushed the connector in. This way you can be sure everything works when you get it back together.
Here's some relevant pictures of mine. Note that I have allen-headed screws. That's because when I got my Ex, it was stolen and the switch was hanging on by duct tape (screws were gone). I tapped the holes and put machine screws in place. Same idea/location for yours though. I think the factory ones are phillips or torx?
Oh, and WHATEVER YOU DO, DON'T BREAK THAT WHITE CABLE that surrounds the wiring for the switch. It may be abnormal in my case (due to UV damage from the surrounding plastic missing for years) but mine snapped in several places. I basically ruined it and was lucky to get a used one (thanks 2stroke!). It's easy to disconnect and swing it out of the way. 1 screw up on top of the column and undo the loop at the end. Just be gentle.
One last thing... I bought a new chinese unit on eBay, think it ran about $50. Has worked great for over a year. I wouldn't suggest paying much more than that anywhere else.
https://dl.dropboxusercontent.com/u/464376/vehicles/1991 Explorer/Pictures/2014-06-26 15.49.55.jpg
https://dl.dropboxusercontent.com/u/464376/vehicles/1991 Explorer/Pictures/2014-06-26 15.50.16.jpg
https://dl.dropboxusercontent.com/u/464376/vehicles/1991 Explorer/Pictures/2014-06-26 15.50.31.jpg
