You can upgrade your OpenVZ VPS server or Shared hosting plan anytime inside your client area .
Go to "My Products" be selecting Menu->Services->My Services and press "View details" button in your service plan row.
Then, from the products menu select "Management Actions->Upgrade/Downgrade" option and select new plan. Upgrade invoice will be generated with the plans prices differences.
After invoice payment your plan will be upgraded.
Important! It is technically NOT possible to directly upgrade KVM VPS Server. If you need bigger KVM VPS Server, then we can create new empty KVM VPS Server. Then you have to transfer all your data/files to a new KVM VPS Server. After that, we will delete your old KVM VPS Server.