Change calendar icon

functions.php

/**
 * This function will change the calendar icon of the Date selection fields in the front-end booking form.
 */
function bkap_calendar_custom_icon_file() {    
    return 'https://www.pngfind.com/pngs/m/681-6816950_small-calendar-icon-png-transparent-png.png';
}
add_filter( 'bkap_calendar_icon_file', 'bkap_calendar_custom_icon_file' );