If you're referring to the flag in the lower right corner of your actual avatar, that is solely based on your IP location. For example, if you use a VPN with a server in another country, it will show that flag.
For the text below the avatar, that is whatever you put in your account profile.
To access that, click on the avatar at the top border of the page, then click on "Account Details" in the pop-up, scroll down a bit, put in what you need.