The easiest way to find the right guide is to run PHPCompatibility with the -s flag:

vendor/bin/phpcs -s

The incompatibility (sniff) code will be in parentheses. Search for it on this page. If you can't find it, then it means that we don't have a guide for it yet. Feel free to submit a guide once you solved the issue in your project, while it's fresh in your mind.

Don't forget to check out the recommended workflow for upgrading your application.