This article will explain how to order add ons for Plesk.
Click on the Manage link under the VPS section.
Click on the Manage button next to your VPS.
Click on the Manage VPS tab.
Click on the Change link under the Billing Details section.
Click on the Yes button to change to Monthly/Annual billing.
Your billing cycle will now be changed.
Please note: The change will take effect at the end of your annual or monthly contract.