On My Macbook Air 2017 (High Sierra, 10.13.6, 2018 005 update), the newly provided Soundtouch app update does not install, sequence of events as below.
Start existing Soundtouch app, and App Update Required is shown, select Update app.
Downloading occurs, Installing starts, when promoted to make changes enter your account password and the updater immediately closes.
To use the existing Soundtouch app select cancel at the prompt where the Mac says Soundtouch app wants to make changes, ignore the oh oh message and continue as normal.
Both my Soundtouch 20 and Soundtouch 300 applied their updates with the IOS app with minor issues:
Soundtouch 300 flashed 2 orange lights after the update until the remote was pressed at which point it could be found and controlled normally.
Soundtouch IOS app prompts for Spotify account details which were previously saved.
Update (21st Nov)
Downloading the Soundtouch app directly from the Support downloads section for a Soundtouch product and running the installer without attempting to remove the application has been confirmed to work, Just remember to stop the Soundtouch Music Server on the Mac first before to run the Installer.