If you installed OTRS on Windows, it is possible to upgrade seamlessly by downloading the new Windows Installer and running it. The installer will automatically detect the OTRS installation and perform an upgrade on it. The installer will stop the OTRS services and start them after the installation is complete, so your users will experience downtime. Make sure you plan some time for the upgrade. Always make sure you have a backup of your server before you start the upgrade procedure!
If you need to install a 'patch level' upgrade (i.e. from OTRS 3.3.1 to OTRS 3.3.2) on Windows all you need to do is to download and run the new installer. Of course you should make sure you have a backup in place before you start the upgrade!
Minor upgrades (from OTRS 3.2 to OTRS 3.3) take a little more time as there is also a database upgrade step involved. The installer will detect this and will automatically perform the upgrade for you. After installation you should still perform steps 12, 13 and 14 of the 'regular' upgrading section.
Please note that if you have any additional packages such as Feature Add-Ons installed on your OTRS system, you'll need to upgrade these as well. Make sure you obtain these before you start the upgrade. After upgrade is complete you can upgrade these packages via the Package Installer Web GUI. Packages from the 'standard' repositories such as Survey, FAQ and ITSM can be upgraded from the Package Manager after the framework upgrade is complete.