A few scenarios are listed on this page:
https://www.mongodb.com/docs/manual/tutorial/enable-authentication/
The most commonly used ones for updating a replica set to enable authentication are below:
https://www.mongodb.com/docs/manual/tutorial/enforce-keyfile-access-control-in-existing-replica-set-without-downtime/
https://www.mongodb.com/docs/manual/tutorial/enforce-keyfile-access-control-in-existing-replica-set/