This is a quick post do get the steps you should follow to patch an Exchange 2013 DAG
1. Run the StartDagServerMaintenance.ps1 script to put the DAG member into maintenance mode, and prepare it for the update rollup installation.
2. Install the update rollup.
3. Run the StopDagServerMaintenance.ps1 script to take the DAG member out of maintenance mode and put it back into production.
4. Optionally, rebalance the DAG by using the RedistributeActiveDatabases.ps1 script.
5. Use this process to install operating system updates from Microsoft Update.