How should I edit the WP Hotel Booking booking form?

It’s personalizing the Howello theme that you buy, and I have a problem with the reservation form.
I do not know how to edit it as edit it so that when making the reservations the clients, if it is already booked, prevent them from continuing with the reservation.
I have no knowledge of PHP, but I imagine there will be a way to do it graphically, without editing the code, and how it is edited so that the reservation system works.
If there is any guide to study me, to be able to make my website with full functionality.
Thank you.