How to do it...

On domain controllers running Windows Server with the Desktop Experience, follow these steps:

1. Sign in to the domain controller holding the Domain Naming Master FSMO role.
2. Open Active Directory Domains and Trusts (domain.msc).
3. In the left navigation pane, right-click Active Directory Domains and Trusts, and then click Raise Forest Functional Level.
   The Raise forest functional level window appears:

1. From the Select an available forest functional level drop-down list, select the desired FFL, and then click Raise.

Alternatively, you can use the following two lines of PowerShell:

Import-Module ActiveDirectory

Set-ADForestMode lucernpub.com Windows2016Forest

Replace lucernpub.com with values for your Active Directory environment.