You have installed a Windows 2008 R2 server Standard Edition after you are finished installing the server you change your mind and want it to be an Enterprise edition.
The first thing that will pop up in your head is I reinstall the server. It only takes an hour.
The easy way is to open cmd.exe as an administrator.
And run DISM /online /Set-Edition:<edition ID> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
To find the different editions run
Dism /Online /Get-TargetEditions
Now the server will restart two times.
Do not do this on a Domain Controller.