Well, before doing so, I think it is best one read this document first - OpenAM 11.0.0 Upgrade Guide.
I find this particular section useful.
Best Practices for Upgrades
1.2.1. Route Around Servers During Downtime
1.2.2. Back Up the Deployment
1.2.3. Apply Customization Before Upgrading
1.2.4. Plan for Rollback
These best practices have been repeated for years, but we do see upgrade failure cases pretty often.
I think most people just read and think they know. However there is no proper thought process going on, which constitutes to failure.
And the worst scenario is there is no Development nor Staging environment to practice prior to Production upgrade! (Yes, this is very true, especially in Asia region. )