It is worth upgrading to the latest version of Zen Cart v1.3.9g ecommerce store to take advantage of the bug fixes particularly if you have a Paypal shipping module installed on your online shop.
The latest release of Zen Cart (v1.3.9g) contains a number of important security fixes so it is advisable to upgrade.
Before upgrading back up. If you have modified any core files you will need to keep a back up of these to avoid deletion in the upgrade process. Of course with the template override system modifying core file is generally unnecessary. Also ensure you back up your database before upgrading. If your store is heavily modified it is advisable to back up all files.
Download Zen Cart v1.3.9g at http://sourceforge.net/projects/zencart/files/
Details of changes to Zen Cart v1.3.9g for this release can be found on the Zen Cart Forum
Also note that there is a mistake in the change log documentation. Two additional files were changed that were not recorded: