Penis color changed from pink to brown, how can I change it back?

This is probably secondary to a contact dermatitis from the deodorant. You should see a dermatologist first to see if this is the cause of the pigmentary change and then there are some very delicate medications that can be used topically to improve the color. Topical corticosteroids in conjunctions with  very delicate skin lighteners can achieve the desired result but you must be under the care experienced  dermatologist who has  seen problem like this before. Don't  use  things like deodorant on a delicate area   such  as the penis