I think that if you have the chrome caps, then it's internal. If so:
Use a screwdriver to pry the chrome caps off the hub. Inside you will find an internal snap ring, in a groove just inside the rim of the hub, there is an external snapring around the axle stub shaft, you will need to remove that too. Now, when you pull the snapring that's aroung the axle shaft, you might need to push the drive "hub" (don't know what it's really called, it's the chunk of metal in there that makes the axle able to drive the hub, splined inside and out) in a bit to get the snap ring off. After both rings are off the spring behind the drive hub will push it out. Remove the splined drive hub, spring, and a thrust washer that should be behind the spring against the wheel bearing. Now, take your selectable hub and stich it in there, put the snap rings back in there. With the one that goes around the axle, you may need to reach around the back side of the knuckle and push the axle outboard a bit in order to expose the groove the snapring goes on. After the snaprings are on, put the hub cap on and screw in the six allen head screws. Then you're done, with the first side. After you finish, you will see how easy it really is.