Download and install the most recent copy of Compass for this relevant package (ie. .exe or .msi or whatever) and version of compass (regular, readonly, isolated).

(This should be able to piggy-back on much of the work in the test install task.)

Run a stubbed update server that will serve up the newly packaged versions of Compass.

Use webdriverio to start that just-installed previous copy of Compass with the HADRON_AUTO_UPDATE_ENDPOINT_OVERRIDE env var that will point it at our stubbed update server.
Make sure Compass updates by waiting for the toast to appear.
Restart Compass and wait for the toast that mentions the new version number to appear.