Upgrading Ucommerce is done with the Ucommerce installer in versions newer than 7.0.
Back-Office and APIs are Automatically Upgraded
When upgrading, use the same install options as when you are doing a first-time installation. Ucommerce automatically upgrades all the files and migrates all database tables, when an existing version is detected. After the installation is done, the back-office and APIs are running in the new version.
Breaking Changes and the Frontend
All that is left to do is to test the frontend for breaking changes.