On the top left of your screen it says “Welcome, goldersgreener”. Click on “goldersgreener”. Then click “Edit” on the top right of the screen. Change it there and click “Update Profile”.
You can get to your profile by clicking on your name at the
top right of the page, underneath the new topic button.
Click on “Edit” on the line of words under your name.
There’s a password-change area near the bottom of that page.