How can you change your commander?

You can change the commander profile image that you use. Tap the image in the top left to access the Commander Info screen. Towards to bottom left is the small profile image with a green circle arrow. Tap this to be able to change the commander image you see and the avatar you use in chat.

If you mean to create or switch to another game commander account (you can play several on one device) go to the Settings menu and click on 'Manage Char.'