Upgrading Mozilla depends on your distribution and it's package management.
On RedHat/Fedora and similar systems with RPM package management use either 'rpm' to install a downloaded rpm file or use 'yum' to upgrade an existing mozilla installation.
On Debian-based systems you will use 'apt' to do the upgrade.
Slackware has it's own way of upgrades, mainly using tarballs.
If everything else fails, go to the Mozilla homepage and download the installer.
Please note that the Mozilla project currently focuses on FireFox as a browser, and the former Mozilla package has been renamed to Seamonkey.
Hope this helps,