What is the content of your file called update ?
I am on an other linux and the content just shows a hint to use the systems package manager to do updates.
The OH2 and OH3 packages have different names ( openhab2 vs. openhab ).
So it is not just an update.
Have a look to Guide: Installing and testing OH3 snapshots/milestones using APT and YUM it explains the most important things.
