Go to personal details
At the bottom, there should be something called change your name

There isn't o-o

Once you click on personal details. Scroll down and it will say User name change.

Go on personal details, on the left side of your screen there should be something like this:
http://gyazo.com/421c58b9077735abbd19324c428eff78 then click on "User name change" and it should bring you to something like this: http://gyazo.com/990796b03a46426730f517f9cf205a29 just fill in the info and yeah :p

oh, i think it wasnt showing because i was on my phone when i was on forums :P