Example of hot to filter the text used for the payment selection header when there is only a single payment method.

example.php

<?php //Please do not include the opening PHP tag if you already have one

function tw_ee_method_of_payment_hdr() {
  return 'Choose Payment Method';
}
add_filter( 'FHEE__registration_page_payment_options__method_of_payment_hdr', 'tw_ee_method_of_payment_hdr');