The CodeIgniter development group has released the latest version of the framework - CodeIgniter 1.6.1:

Version 1.6.1 is primarily a maintenance release, but does bring a handful of nice feature additions and enhancements. After a very successful 1.6.0 release, a series of bugs have been squashed and enhancements have been made that we wanted to roll out as a formal release.

The update process is just replacing a few files from your current installation. Check out the Changelog for more information on the updates made and, if you're not quite sure what you're doing on the update, check out their handy update instructions.