How do you become carnage on ultimate Spiderman xbox?

You cannot become Carnage in Ultimate Spiderman on Xbox. There is no option to transform into Carnage, nor are there any cheats or mods available to do so.

The game focuses on Spider-Man's journey and the story does not include Carnage as a playable character.

Perhaps you're thinking of another game, like Spider-Man 2 (2004) on PS2, where you could temporarily control Venom and fight as him.