Install SQL Server 2017 - CU21 on Distributed availibility group (DAG) environment

Hi All,

I have to install SQL Server 2017 - CU21 on Distributed availibility group (DAG) environment. Could you please share the steps to install CU21 on all 4 DB servers.

DC1:
SQLDB1 (Primary)
SQLDB2 (Secondary)
DC2:
SQLDB3 (Forwarder)
SQLDB4 (Secondary)