Deleted

If it were me personally, I'd just delete the GA. Yeah you lose a slot, but it was an honest mistake. The devs will note in the deletion record that it was a mistakenly created. If anyone complains, ignore them. These things happen sometimes.

It does change the game being given away, but you can't just change it to any random game, it would likely not be approved. Usually it's used to change the giveaway to a different edition of the same game, or in some cases if 2 games have same or similar names and the creator confused them.
What I would do in this case would be deleting the giveaway, then recreate it as XCOM: Enemy Within