This page gives the directions how to upgrade your Debian distribution
How to upgrade your distribution
Upgrading from one stable release to the next (e.g. Lenny to Squeeze) is done by following the release notes for your architecture. For most people with 32 bit systems that means the Release Notes for Intel x86. For most with 64 bit systems that means the Release Notes for AMD64.
Minor upgrades are done with the package management tools: