Thanks to a recent update you can now change your avatar on Pokemon Go whenever you feel like you need a new look. Before, you could only choose customizations for your character at the start of the game.
To access the customization menu, tap on the face of your avatar at the bottom of the map screen. Then, tap on the green menu button on the lower right-hand side of the screen and choose Customize from the list.
Once you have the customization menu open, you can change almost everything about your character. Your choices are skin color, hair color, eye color, hat style, top style, pants style, shoe type and backpack type. You can even change gender if you like.
Just tap on the icon of the item you would like to change and use the arrows at the bottom of the screen to scroll through your options. Once you find what you're looking for, tap on it and it will be placed on your avatar.
Don't forget to tap on the checkmark button on the bottom right-hand side of the screen to save your choices. If you back out of the customization screen without clicking this button all of your changes will be lost.