This is probably the part that will crash your site, so proceed with extreme caution! Migration actually starts here. Unlike the previous steps you will not be able to use drush to overwrite your core to Drupal 7. This has to be done manually. You have to download the latest stable version of Drupal 7 and paste into the root folder of your website. Make sure to not overwrite .htaccess and robots.txt. When you overwrite, merge into the folders and replace the files. That being said, there are some prerequisites to this step.