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