This phase of the process involves code related task efforts. You will need to analyze the libraries and dependencies that you need to update, and of course, all the coding refactor needed because of the upgrade.
Something that worked for me is to upgrade my local to 6.5 and then upgrade the code. Doing this I was able to identify dependencies that I was missing from my initial assessment.
Keep in mind that (at least in my scenario) there were many changes at AEM libraries, methods were deprecated, how to access the repository through code changed to have stronger security, which means that having a good idea of what your solution does, and doing pretty good testing is a key factor that will allow you to have a better outcome of this process phase.
The substeps within this process are:
Create a dedicated code branch
Cleanup content in production
Backup and roll down content from production to lower environments.
Align repository structure with 6.5