How can I change my character's appearance after starting Pokémon GO?
As of version 0.31.0 you can now change your avatar's appearance by navigating to the new menu in your profile and tapping the "customize" option.
>At this time, it is not possible to change your avatar or your team.
Cited from here
Since version 0.31, you can re-customize the avatar from the Trainer profile screen (click the right bottom menu).