As you explore the island of Solstheim, you'll find Black Books scattered around that grant great abilities. One of the books, the Waking Book will allow you to reassign your perk points after completing the dungeon you enter when reading it. The easiest way to find this point is to complete the main quest line of the Dragonborn DLC.
After the final battle of the quest line, you'll be allowed to reassign your perk points. However, after you acquire the book, you can use it any time.


