How to change texts set in php files

The easiest way to change phrases set in php files is to use a plugin like Say What.

https://wordpress.org/plugins/say-what/

After you install and activate the plugin, head to Admin -> Tools -> Text changes to start using it. When adding a new text change, set the text domain field to bookyourtravel and leave the text context field as empty.

Btw, in order to find the exact text phrase to change, we recommend you use the free version of poedit to open the theme's languages/BookYourTravel.pot file and use ctrl-f to find the phrase you are looking to change.