By default, the plugin uses the text provided by the Facebook API, but this can be changed by adding the following snippet to the plugin’s “Custom JavaScript” section (Customize > Misc):
$('.cff-cta-link a').each(function(){
if( $(this).text() == 'Book Travel' ) $(this).text('Book Now');
});
This will change the button text “Book Travel” to be “Book Now”, for example.