To upgrade server to server 2012 R2, it requires windows server 2003 and higher DC
Insert OS installation disc and go to folder source\ADPrep\ADPrep.exe
Run ADPrep /ForestPrep in forest and
ADPrep /DomainPrep on each domain
Once complete, run command RepAdmin /SyncAll /APeD – (force a sync to all the document controllers in domain) wait few minutes until domain syncs.