First word of wisdom don't.
Why not? Because 1.11 is insecure and no longer supported. You would want to upgrade to 1.10.6 or 1.12 (current stabled) as those are the two currently supported versions. Between the two I would recommend 1.12.
Secondly check the system requirements and comparte them to your system. Generally you really only need to concern yourself with your PHP version and make sure that it is 5.6+. 7.2 is currently the recommended version for 1.12.
Third make sure that you back up your database and site files BEFORE you do the upgrade. While I haven't encountered any large problems in the years and years that we have been running osTicket some times stuff happens [I've been running osTicket since 1.6].
Fourth it's usually cleaner to delete your sites files and install the new ones [then restore the /includes/ost-config.php with your settings in it]. Some times files get changed and the upgrader does not remove old un-necessary files.
Lastly, I always clone one of my production sites and create a temporary test site to perform the upgrade on it first. This has largely been an un-necessary step but I find value in running test and production versions of my sites. This also means that I have place to test out new plugins, mods, etc.