On a minor release such as the 7.2 -> 7.2.1 upgrade it's fairly simple.
Download the zip file. Extract it. Close your forum. Upload the entire install directory and also the libs/ubbthreads.inc.php file.
Browse to install/upgrade.php on your forums and enter your username/password. You'll get a list on that screen of what files you need to upload, there are only a handful. Once you have them uploaded, continue through the rest of the upgrade script. Once it's complete, remove the entire install directory.
