Navigate to the VPS tab in your control panel and select Your VPS > Big Storage. On this page you can order new disk space for your VPS using the Order Big Storage button.

You can determine the size of your Big Storage by pressing the '-' and '+' symbols and complete your order.

After your order has been completed you will return to the Overview page. Here you can find your ordered Big Storage. Select it and you will see the option to Attach your Big Storage to your VPS. Select your desired VPS in the dropdown menu and click "Attach Big Storage to VPS" to link it to your VPS. After you have linked the Big Storage to your VPS, open the Console in your control panel.

Log in as Administrator on your Windows Server VPS and open the Server Manager. In the Server ManagerDashboard you will find the option Tools > Computer Management on the top right of your screen. Next, select Storage > Disk Management. Under Disks you will now find an extra disk; Disk 1. This will have the status Not Initialized. By right clicking the disk with you can select Initialize Disk.
When the disk is still marked as Offline you can right click the disk and select Online.

The disk is now online on your VPS and all that is left to do is to link the disk. For this, right click Unallocated and select New Simple Volume…
The system will open a Wizard which will guide you through the steps that will allow you to give the disk volume a name (like E:). When you have completed the Wizard you are ready to use the extra disk space.

When you navigate to This PC in you File Explorer you will see that Big Volume has been added to your VPS.