Could you describe what it did? For my own service install, update Install is usually OK. Activate has a known issue (which has so far resisted repair) which activates the old version (workaround is simply service restart).
What version does it show in About → System Info? You’ll see BaseVersionName and ServerVersionName. The reason there are two versions is because the base looks around for any installed update it should run.
Downgrading / reverting to a lower version describes the plan. If you find your failed update folder (it should be named 2.0.4.5) looking emptier than the Program Files base version, that would keep it from being used.
I’d agree that Duplicati running as a service keeps its configuration in a different place by default, although the plan is actually the same. What changes is the user, which is SYSTEM by default, so Duplicati is under C:\Windows\System32\config\systemprofile. After Duplicati gets back up, it would be best to use the Export GUI option to save any configurations you want to be sure are preserved. They’re not automatically saved, and Windows has a bad habit of cleaning out the SYSTEM profile during its twice-a-year version updates…