Migration guides contain information regarding the new features introduced in each version and the migration path between 5.x minor releases.
CakePHP provides an upgrade tool that automates many code changes using Rector. The tool has rulesets for each minor version to help automate tedious code changes like method renames and signature updates.
To use the upgrade tool:
# Install the upgrade tool
git clone https://github.com/cakephp/upgrade
cd upgrade
git checkout 5.x
composer install --no-dev
# Run rector with the desired ruleset
bin/cake upgrade rector --rules cakephp51 <path/to/app/src>
Run rector before updating your composer.json dependencies
to ensure the tool can resolve class names correctly.