Some of the individual tools within BackTrack (such as Metasploit) have their own update methods, but to update the versions of software installed from the repositories you can run the following commands at the prompt. This will update the apt database from the BackTrack repositories and update any installed software that has a new version available in the repositories.
apt-get update
apt-get upgrade
Please note that it is generally not a good idea to update any software in backtrack via any other means than via our own repositories.