To change your Gravatar, you go into your account page, as if you were going to edit your profile.
Scroll down a little to the box called "Avatar" in there you can select the option "Show My Gravatar"
The word Gravatar is a link. When you click on it it will take you to a different page where you simply sign up, and then upload a picture from your computer or the internet to use as your gravatar.
Let me know if there's something you still don't understand.