Hi,
I'm the newest member for several hours now, so let me ask these questions before they start to become ridiculous.
It doesn't seem easy to avoid bringing in issues that are clarified elsewhere.
You can't currently change your username afaik. You could deactivate and create a new one. But probably the "admins" can do it in the database :)
You change your avatar by adding a picture to https://gravatar.com/ for the email you used to register in the forum.