How do you change Batman's costume in "Batman: Arkham City"?

Hello, i have batman arkham city, and i beat the game and it told me after the credits that i could change batman's costume in story mode, but i cant figure out how. It never asks me if i want to change it and theres no option to in the game at all. i havent finished all the side missions yet, im still working on the riddler one. could this be why i cant change the costume? thanks!How do you change Batman's costume in "Batman: Arkham City"?
you need the dlc or code to download the character skins. Once you do that you can switch into batmans new outfits.

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