This article will show you how to change the DAG witness server on Exchange Server 2013.
To change the DAG Witness Server on Exchange Server 2013 we need to use the EAC -> Servers ->Database Availability -> Edit
On the DAG edit menu change the server name and the witness directory folder.
If you are getting the error message blow check the article in the link:
“The Exchange Trusted Subsystem is not a member of the local Administrators group on specified witness server”