With the updated set, players can change their skills to legendary, which resets the skill to level 15. This returns any perks and allows the skills to be affected by leveling once more. Additionally, the update adds a new legendary difficulty setting.
Several bugs are also addressed by the patch, including a glitch that would cause dragon nemesis Alduin to become invincible and improper NPC respawns. You can read the full list of changes here. Update 1.9 was released for Steam users in March.