Set a custom header to replace the default "Membership Required" heading for the PMPro non-member message.

custom-no-access-message-header.php

<?php
/**
 * Customize the no access message header.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_custom_no_access_message_header( $header, $level_ids ) {
	$header = 'Your Custom Header Here';
	return $header;
}
add_filter( 'pmpro_no_access_message_header', 'my_custom_no_access_message_header', 10, 2 );