Steps for downgrade Domain and Forest functional levels
1.Currently im running with 2012 forest and domain functional levels
2.Using set=ADForestMode -ForestMode Windows2008R2Forest PowerShell command we can downgrade the forest functional level
3.Now we can see from GUI, its downgraded.
4.Using Set-ADDomainMode -DomainMode windows2008R2Domain PowerShell we cn downgrade the domain functional level also
*I have tested this in environment where there are no ad dependent application such as Exchange or SharePoint.