To access the UCP, click your name on the upper-right hand corner of the forum. A drop-down menu will give you some options including the UCP.
From the UCP you'll have many options including signatures and avatars. To change your avatar, click Profile in the menu which will take you to a new panel. From there, you'll see a drop-down with two options: Upload Avatar, if you want to use your own, or Gallery Avatar to select one I've uploaded to the forum.
