Just log into Xbox Live and you'll be prompted for your update. The update fixes the following:
-Fix: the game no longer freezes during the Sassan summoning scene
-Fix: NPC characters can no longer disappear
-Fix: various cut-scene/dialog issues have been addressed
-Fix: various story and gameplay/balancing issues have been addressed
-Fix: no longer able to start a new game before the character model is loaded in
-Feature: by popular demand, a Nightmare difficulty mode has been added to provide players with the ultimate challenge!