To upgrade your VPS, please follow the steps below:
1) Login to the Quantum Core Client area HERE
2) Once logged in, hover your mouse on "My Account" located at the very top of the page and click on "My Products" (alternatively, click HERE)
3) Select the service you'd like to upgrade
4) Click "Upgrade/Downgrade" from the left side options
5) Choose the new plan and billing cycle and select "Choose Product" on the corresponding plan
6) Once the upgrade request has been paid for, it will be automatically processed.
*Note: Your server may automatically reboot to perform the upgrade