First, back up the main site-specific folders elsewhere: sites, modules, themes, profiles. Then delete entire contents of original directory. Run composer create-project, which fills directory with composer-specific files and creates a web subdirectory. Copy the backed-up folders into web. Reset permissions recursively to www:www. Create website entry in apache as a VirtualHost, point to web subfolder. Add DNS entry for site as CNAME to host. Add DNS entry to main DNS provider's records. Set .htaccess. Launch. Run status report and correct any errors.