This guide will show you how to migrate a site that meets the following criteria to the latest version of Drupal:
|How Site Was Created
During this process, you will work in a new branch to replace the entire file structure with the code from Pantheon's Integrated Composer upstream, and then re-add your contrib and custom code to the new codebase. Then, you will create Multidev environments on individual sites for testing and to apply any site-specific code customizations.
If this is not your scenario, see Drupal Migration Guides for additional upgrade paths.