To upgrade Windows 8 single language to Windows 8 Pro, follow the steps given below:
1. Right-click My Computer icon on the desktop.
2. From the displayed context menu, click Properties.
3. On the opened System Properties box, click Get more features with a new edition of Windows.
4. On Add features to Windows 8 box, click I want to buy a product key online option which redirects you to the screen from where you can purchase Windows 8 product key, and can upgrade your Windows 8 standard to Windows 8 pro instantaneously. Alternatively you can also click I already have a product key option if you already have a Windows 8 product key.
Hope this helps.