Add the sample code below to your child theme's functions.php file or in a site specific plugin. Then change REPLACE ME to the new messaging that you would like to display to your attendees/registrants.
<?php
//* Do NOT include the opening php tag
//* Adjust the pre-approval messaging in registration checkout
add_filter ('FHEE__EE_SPCO_Reg_Step_Payment_Options___events_requiring_pre_approval__events_requiring_pre_approval_msg', 'ee_custom_messaging_pre_approval_registration_checkout');
function ee_custom_messaging_pre_approval_registration_checkout(){
return 'REPLACE ME';
}
References:
https://eventespresso.com/topic/changing-system-notifications/
http://eventespresso.com/topic/changing-applications-from-not-approved-to-pending/