You can just download the latest version on AD connect and run the set up file on the servers. This will automatically update AD connect to latest version. You will also get the customizations in place that you already have in older AD connect.
Recommendations:
I would recommend you to perform the upgrade process first in passive server. Once updated to latest version you can make passive server as active and active server as passive. Post this you can update the other AD connect also to latest version.
You can also refer below article to get more information regarding this.
Let me know if you have any further questions.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.