SMF provides three types of packages that you can download; the install, the upgrade and the update packages. The concept behind the install package isn't difficult to understand, but the difference between the upgrade and update packages can be confusing. Both packages have been designed to bring your SMF installation up to date from a previous version of SMF, but there are some subtle differences.

The Update Package:
This package contains all the files needed to update your installation of SMF from the previous version, but may not work completely with older versions. It only contains the files that have been changed since the last release of SMF, along with the upgrade.php script. For example, the 1.0.9 update package should be used to update only from 1.0.8.

The Upgrade Package:
This package will upgrade from any previous version of SMF. It contains all SMF files except for Settings.php and install.php, and also includes the upgrade.php script. For example, the 1.0.9 upgrade package can be used for SMF 1.0 through 1.0.8, along with any beta release of 1.0 and even can be used to upgrade from YaBB SE 1.5.5.

If you are upgrading with a lot of installed mods, moving from YaBB SE to SMF, upgrading from one major version to another (i.e. SMF 1.0 to SMF 1.1), or if you simply aren't sure which package to use, then the upgrade package is the recommended package to use.