If you have a previous release of Übercart already installed and you need to upgrade to the latest version run the following steps.

1. Go to http://drupal.org/project/ubercart or http://drupal.org/node/129292/release, download and untar / unzip the latest release on your local system.
2. Backup your existing Drupal database and installation directories / files (just to be on the safe side).
3. IMPORTANT AND OFTEN FORGOTTEN - Run the Drupal "update" script. You do this by typing the following url into your browser "yoursite.com/update.php" (of course you need to replace yoursite.com with your actual site! ). If you see the error message, "database schema out of date" this is Drupal's reminder that you need to run update.php

Although this is pretty simple, sometimes people forget to do that last "update" step.