To set your flag, first tap your profile name at the bottom left corner of the screen.
Then tap the small Ace of Arenas icon next to your user name in the top section of the screen.
This will take you to the flag selection menu.
You can scroll left and right through this menu to view more flags. You can also tap on any of the letters at the bottom of the screen to display only the flags of countries beginning with that letter.
To select a flag, tap it and then tap the Back button at the upper right corner of the screen. You will be prompted to confirm your change.