If you click on the profile button that you see just underneath your user info dialogue box at the top of the screen, you will find the page to edit your profile details.
On the left is an option called forum profile information - click on this.
There are 2 options on this screen that you can complete:
Personal text - this is displayed just underneath your avatar. I've typed Hi everyone! on mine.
Custom title - this is displayed just underneath your username. Mine reads Potty training.
When you've finished, scroll to the bottom of the page and click on change profile to save the changes.
HTHs
Rach xx