My Top 5 Tips to Get Ready for Upgrading to Rails 5.2.

Ensure as much test coverage as possible
Upgrade to the latest 5.1.x release (step through each major release if you are on earlier versions such as 5.0.x or 4.x)
Fix All Deprecation Warnings
Setup error tracking (Rollbar, etc)
Use railsdiff.org to help you upgrade your files correctly