Did that page ask for you to log in? If you don't have www. in front of the url when you log in and click a link that does have it, you might not be logged in when viewing that page.@Ali. and @MattW its saying you do not have necessary permissions to change your username. I think its becoz me, @A.R123 Max and @Kashif567 share the same internet????
Your account does have permission to change it, and I again ask that you check the menu named 'Haseeb22' next to the inbox - and take a screenshot of that menu if you cannot find the 'Change Username' option - it should be the first one on the left.