Assuming that you finished the game (destroyed the *snip* *snippity snip* ) You should just be able to go there, and have at it. Unless you killed them all, then you'll have to use an older save game/start over. As far as I know there's no way to edit quests in/out of a savegame.
*I intentionally snipped out the spoilers, there's been lots of new people and I don't want to spoil it for them.