So you installed Windows Server 2012 Standard by mistake and want to upgrade to Windows Server 2012 Datacenter?

Easy! Use powershell:

DISM /Online /Set-Edition:ServerDatacenter /AcceptEula /ProductKey:AAAAA-BBBBB-CCCCC-DDDDD-EEEEE

Use the following to get your current version:

DISM /online /Get-TargetEditions

What’s the technical difference between Windows Server 2012 Standard and Datacenter? Nothing! See Product & Licensing. Cut’n’paste:
Q2: What is the difference between Windows Server Standard edition and Windows Server 2012 Datacenter edition?
Both Standard and Datacenter editions provide the same set of features; the only thing that differentiates the editions is the number of Virtual Machines (VMs). A Standard edition license will entitle you to run up to two VMs on up to two processors (subject to the VM use rights outlined in the Product Use Rights document). A Datacenter edition license will entitle you to run an unlimited number of VMs on up to two processors.

Also check the Microsoft Product and Comparison guide which shows you the same thing.

And remember, once you go to Datacenter edition, you can’t go back!