Adding and editing email notifications is managed in two areas depending on which plugin are you using.
Formidable: https://formidablepro.com/knowledgebase/email-notifications/
and WP Booking System:
http://bellevuetheme.com/2015/09/09/wp-booking-system-create-a-booking-form/
more about WP Booking System...
http://bellevuetheme.com/2015/09/09/wp-booking-system-receive-and-manage-bookings/
http://bellevuetheme.com/2015/09/09/wp-booking-system-publish-a-calendar/
http://bellevuetheme.com/2015/09/09/wp-booking-system-manage-availability/
http://bellevuetheme.com/2015/09/09/wp-booking-system-create-a-calendar/