2008 to 2012 server upgrade error

I am trying to upgrade windows server 2008 R2 to windows server 2012 R2 datacenter. It give me below mentioned error:

Windows Server 2008 R2 Datacenter cannot be upgraded to Windows Server 2012 R2 Datacenter (Server with a GUI). You can


choose to install a new copy of Windows Server 2012 R2 Datacenter (Server with a GUI) instead, but this is different from


an upgrade, and does not keep your files, settings, and programs. You’ll need to reinstall any programs using the original


installation discs or files. To save your files before installing Windows, back them up to an external location such as a


CD, DVD, or external hard drive. To install a new copy of Windows Server 2012 R2 Datacenter (Server with a GUI), click the


Back button in the upper left-hand corner, and select “Custom (advanced)”.

Need help on what need to done