You can upgrade or downgrade your hosting package at any time from within the client area.
Log in at http://www.fast2host.com/clientarea.php
Select "My Products & Services" from the menu on the right, and this will display your current package(s).
Select "View Details" to the right of the package. This will display details of your current package.
Scroll down and click on the "Upgrade/Downgrade Package" button. You will then be presented with a list of possible upgrade or downgrade options.
Select your new package. You will then be presented with the payment details for your upgrade. Select your payment method, and click on the continue button. You will then be presented with a link to your invoice. Once this is paid, your package will be upgraded.