Change the button text for one event, Event Espresso 4.

change_button_text.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter( 'FHEE__EE_Ticket_Selector__display_ticket_selector_submit__btn_text', 'my_custom_button_text', 10, 2 );
function my_custom_button_text( $btn_text, $event ) {
	if ( $event->ID() == 51803 ) { // change this ID to match your event ID
		$btn_text = 'Get on the bus!'; // change button text here
	}
	return $btn_text;
}