Upgradation of Volume license of Windows 8 Pro to Windows 8.1

Hello,

My organization purchased a Volume license of Windows 8 Pro.

Windows Prompts for Upgrade to Windows 8.1 but when clicked on it. Windows Store gets open and nothing happens.

I gone through some of the Microsoft link and found that Volume license cannot be upgrade from Windows Store.

Kindly let me know the step by step process to upgrade Windows 8 Pro to Windows 8.1 and use its feature.