When upgrading from Tableau Server 2018.2 or later, you do not uninstall the previous version. The upgrade process is designed to install a new version side-by-side with your existing version, and then switch from the earlier version to the later one.
When doing a TSM-to-TSM upgrade (version 2018.2.x to later version), you must leave your existing version in place and running until just before you upgrade using the upgrade script. If you uninstall your existing version before upgrading, you will not be able to upgrade. For your end-users, this has the impact of reducing downtime because you install your new version while the existing version is running. The only time the server is not running is during the period the upgrade script is actually upgrading to the new version.
After installing the new version and upgrading, you can uninstall the previous version when you choose to do so. You can leave the older version installed indefinitely, though one reason you might choose to uninstall it is to free up disk space used by files specific to the older version. To understand the difference between uninstalling Tableau Server and completely removing all aspects of Tableau Server, see Uninstall Tableau Server.
Upgrading Tableau Server requires a stop and start the server as part of the upgrade process. During this stop/restart Tableau Server is unavailable.