The necessary stepts are as follows:
– Sign in to Xbox LIVE then press the guide button
– Go to Settings->System Settings->Memory.
– From here, scroll down to Fall Out 3 and press A.
– Select Fallout 3: The Pitt (Marketplace Content) press A then choose Delete, then Yes to Delete.
The Pitt DLC pack can be downloaded once again at no charge. The pack costs $10 and includes new quests, items and new area to explore: a post-apocalyptic version of Pittsburgh.