my_pmpropp_level_checkout_gettext.php

<?php // copy form below.
/**
 * Change the "Select a Payment Plan" heading for a specific level
 *
 * 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_pmpropp_level_checkout_gettext( $pmpro_level ) {
	// Add gettext filter on level 1 checkouts only.
	if ( '1' === $pmpro_level->id ) {
		add_filter( 'gettext', 'my_pmpropp_change_plans_heading', 10, 3 );
	}
}
add_action( 'pmpro_checkout_preheader', 'my_pmpropp_level_checkout_gettext' );

function my_pmpropp_change_plans_heading( $translated_text, $text, $domain ) {
	if ( 'pmpro-payment-plans' === $domain ) {
		switch ( $text ) {
			case 'Select a Payment Plan':
				$translated_text = 'Select membership duration';
				break;
		}
	}
	return $translated_text;
}