User blog comment:Jimeee/Skyrim 1.9 beta update adds Legendary difficulty and raises level cap (Updated Apr 8th)/@comment-7812306-20130409162711/@comment-198.91.10.183-20130409163442

There are two ways. If you have the Dragonborn DLC, you have to complete a quest...check out the wiki article for it (I won't post spoilers here).

The other way is to raise the skill to 100 first. Then go into the skill tree and press "Y" to make it a "Legendary" skill. The game will return all of your perks in that skill, and reset it back to 15 so that you can level it up again.

Hope this helps!