You can download any sqlite browser utility, such as this one: https://sqlitebrowser.org/
Open “users.db” and delete this user:
https://i.gyazo.com/9e7816d2f93ff67d3856e8901552018e.png
Open deployer/index.php and change line 41 from private $allowRegistration = FALSE; to TRUE;
Go on the website, you now will have a “register” button. Register your new admin, then revert $allowRegistration back to FALSE.