The part that the rubber boot sits on needs to be removed. I think there is a spiral retaining ring, looks like the end is at about the 3 o'clock position in your picture #2.
To remove those, you get a small screwdriver under the end tang, and wedge it toward the center, once it clears the groove, it will try to lift out, and you just spiral out the rest of it. Installation is just get the first part in, and squeeze / spiral into place.
Here a video on removing / installing those:
https://www.youtube.com/watch?v=ka6jmXWqmoY