the simplest solution to this is delete your wp-config.php in your root folder it should ask to reinstall again
@oldglow, i see no problem if you give them your admin credential, you make them do whatever thing they need to do, I believe this is a new site if they are done, at the end of the day just change all the credentials if you don’t want them to work again, but if you want that person to continue working/maintaining your site,give them another admin account + ftp they need it both
