Windows Server 2012 R2 functional level

You have a domain running with five domain controllers (four running Windows Server 2008 R2 and one running Windows Server 2012). The domain is now running the Windows Server 2008 forest functional level and the Windows Server 2008 R2 domain functional level. What do you need to do to make the domain and forest Windows Server 2012 R2 functional level?

the references most include references at the below:
references

https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/upgrade-domain-controllers-to-windows-server-2012-r2-and-windows-server-2012?redirectedfrom=MSDN

http://www.mustbegeek.com/understanding-gpo-in-windows-server-2012/