Upgrading the Zabbix frontend from one minor to another minor version should be simple as well. If installed from the sources, copy over the new frontend files. Instead of overwriting the frontend, it might be a good idea to copy the frontend to a separate directory first, verify that it works as intended, and then move your users over.

For example, if your original installation had the Zabbix frontend in the relative path, zabbix/, place the new frontend files in zabbix-<new_version>/, rename the zabbix/ directory to zabbix-<old_version>/, and create a symlink called zabbix that points at the new version so that you don't have to use a different URL whenever you upgrade. To skip the configuration wizard, copy over the configuration file:

# cp zabbix-<old_version>/conf/zabbix.conf.php zabbix/conf/  

That should be enough. Now, you can refresh the Zabbix frontend in the browser and check the page footer; the new version number should be visible there.

This approach with keeping the old frontend versions is useful if a new version turns out to have a problem and you would like to test whether the old version also had the same problem; just load up a URL in your browser that points to the old frontend directory. If the problem indeed turns out to be a regression, simply change the symlink to point to the old directory and revert to the old version.

You may keep and use multiple versions of the Zabbix frontend in parallel, as long as they all are of the same major version. While normally not needed, it can be very helpful when some debugging or comparison has to be performed.