So you've decided to upgrade your site to Drupal 6! This section is meant to guide you through that process, after having gleaned from both experience and the best advice out there the concrete steps you should follow. The eight steps involved in any upgrade are:
Shift everything over to a test site (you may already have one, but this is just to make sure you are acting on the real McCoy, just in case you are not using version control).
Update everything to the latest Drupal 5.x version available, not only for Drupal core, but also for all the modules you are using.
Test everything running in the latest and brightest 5.x version.
Make a module inventory, and see which modules might not yet have been upgraded to a Drupal 6.x version. The modules that you have installed are the most important factor in determining how easy or difficult, or even possible, the upgrade process will be.
Switch to the default Drupal theme. Research whether or not your theme is available in a version for Drupal 6.
Disable all modules. Now, do not uninstall any of the modules — you don't want to disturb your data. You just want to leave the field clear for the Drupal 6.x core database upgrade to take place first.
Update the Drupal core.
Update each of the modules and the theme.
Pinch yourself: It's alive!
However, even though each of these steps will always be present, they may vary in significance and the amount of work involved and the problems encountered in each case. ...