To change your avatar:
-tap "You" and select "Settings"
-in the General settings, find the Avatar row
-tap "Choose file" and select a picture from your files
-tap "Save Changes"
Your new avatar may not appear immediately to you and others. If you wish, you can try refreshing your page to see your new avatar. If it still doesn't change, clear your cache. (Google "clear your cache" if you don't know how.)