You can change your avatar in profile section. Here is how it can be done:
- Tap on your nickname in the upper right corner on the main screen in game. You will get to your profile.
- Tap on "Avatar".
- Choose an available avatar from the list that you want to equip and tap on it.
You can unlock new avatars by collecting their pieces. Once you have all of the pieces of avatar, you will be able to equip it.