Tap the top left navigation menu (hamburger menu).
The menu will open by sliding out from the left.
Tap the down arrow, located to the right of your Squad name for extended Squad selection options.
Upon tapping the arrow, the Squad list will drop down. Scroll down to the bottom of your squad list until you see the 'Edit Squad' cog.
The 'Edit Squad' module will appear. From this module, you can tap on 'Edit Avatar' to edit the existing avatar image or edit the Squad name. Additionally, you can leave the squad from here as well.