Theme upgrades occur very infrequently, and are rarely required for security reasons. The Upgrade status module will tell you if there is a new version of your theme available. However, if you have made changes to your theme by modifying the actual files or style sheets within the theme, note that an upgrade will eliminate your changes unless you specifically preserve them. In short, although the theme should be upgraded if there is a security problem (which is extremely rare), themes generally don't require upgrades.

Should your theme require an upgrade, you should download the new version of the theme, and look for any upgrade instructions within a readme.txt file or an upgrade.txt file. In the absence of any other instructions, delete the old theme directory, and upload the new theme in its place.