It's actually a tad tricky. I had to fart around with my settings for a while to figure it out. Anyway, first, go up to the bar at the top of the screen and click where it should say "Welcome Rainfeather". From there, click the option that says "Edit Profile". I think Rea has edited the link color, so now the tabs are a little wonky (nothing is showing up), but if you click on the fourth tab from the left, you'll have a whole host of options for avatars. Make sure you either have an image URL of the avatar you want ready at hand, or select a premade one from the gallery. Or you can use the create option, but I believe that's technically off-site, and I haven't used it personally.
Hope this helps! c: