To change the billing address online you would need to be logged in as the Account Owner Role. Only the Account Owner Role can change the billing address online.
On the main page when you first log in as the Account Owner, you can just click on the the change address link to the far right under Account Actions in the I Want to box. There you can proceed to change the address.
You can also hover the mouse arrow over the Account Tab, then click on the Account Profile Link, then you would go to the Billing Information and click on edit below.
To the far right under the Account Actions in the I Want To box, you can also choose the change address link there as well.