Hi Mari
Microsoft recommends to upgrade the mailbox server first and then the cas server from Exchange 2013.
For servers in DAG first update the passive copies.Ensure that you don't have any active copies mounted on the passive server.Keep the server that you are going to upgrade in DAG maintenance mode.
Ensure that you don't have any backup and virus scanning running on the server that you are going to upgrade as it can cause any issues.
Cheers !!!
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