How to I edit or change my character’s avatar?

The first time you play, you will be prompted to set up your characters avatar. If you wish to change it in the future, go to the screen which shows you team before a mission (selecting any mission will do); select your character; in the bottom left of the dialogue box that comes up you will see the below icon, select that, and then edit as you wish (be sure to select the tick after you are happy with your changes).