You will need to a hard reset or use the windows device recovery tool.
If you do use WDRT it will flash WP8.1 on it and you will need to use upgrade advisor to get back onto WM10 but even then it will be the anniversary update at max. You will still get security updates and some bug fixes but that's it.
So your best option is to do a hard reset as the preview programme for phones is no more.