I don't believe you have the option of changing back to auto-aim from manual in HUT. You should be able to do that from the Advanced Gameplay options menu, but that is disabled in both the pre-game and in-game menus.
The only way to play with auto-aim in HUT, that I know of, is by playing on Rookie difficulty. Unfortunately, this does NOT include the Rookie Challenge tournament. The Rookie Challenge tournament still uses manual shot aim.
Either do a "Play Now" outside of the tournaments or challenge the team of the week and select Rookie as the game mode. Alternatively, using the glitch detailed in these forums (link here
) you can play on "Superstar" difficulty while having many of the Rookie gameplay settings, including auto-aim. You aren't able to just enable auto-aim solely so all of the applicable settings will be changed.