Your account defaults are the pieces of information that are most likely not to change from order to order. They include your shipping address, billing information, password, and contact information.
If you need to change shipping or billing information for just one order, you can do that during the checkout process. The information will be changed for that order, but will not be saved in your Account Defaults profile. If you want to make a permanent change, go to "my account", and choose "Account Defaults" from the submenu. Change any necessary information, then be sure to click "Update" to save the new information.
Changes will be reflected immediately in your next order.