There are several important steps in upgrading a Drupal site to Backdrop CMS.
Step 1: Research what will be necessary to upgrade to Backdrop CMS
The first step is researching what will be necessary to upgrade your Drupal site. This is most likely the reason you are here. We've provided a list of items to review on the Drupal 7 site that will help inform how much effort it will take to convert the site to Backdrop.
Step 2: Prepare the Drupal site for an upgrade to Backdrop CMS
The second step is to clean up the Drupal 7 site. This should include removing any content or features that your team is no longer using, as well as removing features that are no longer supported by Backdrop. We've provided a list of things to prepare, and some more information on what to do if you were using a module that has been removed from core.
Step 3: Upgrade your Drupal site to Backdrop CMS
The third and final step us to run the update script (update.php). Since there are some significant changes between Drupal and Backdrop, we recommend keeping a copy of the old Drupal 7 site up for comparison after the upgrade. We've provided some detailed steps on running the upgrade to help guide you through the process safely.