Fix the billing amount shown on a user's account with Paid Memberships Pro

my_fix_billing_amount.php

<?php
/**
 * Change the billing amount and cycle number/period for a user's active membership.
 * This is useful if the number is out of alignment with the gateway.
 * Change the variables under "change this stuff"
 * Then visit /wp-admin/?my_fix_billing_amount=1 as an admin.
 * Deactivate this snippet after running.
 */
function my_fix_billing_amount() {
	global $wpdb;
	if ( ! empty( $_REQUEST['my_fix_billing_amount'] ) && current_user_can( 'manage_options' ) ) {
		// change this stuff
		$user_id = 31;
		$billing_amount = '3';
		$cycle_number = '1';
		$cycle_period = 'month';
		
		// find their memberships users entry
		$sqlQuery = "SELECT * FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . esc_sql( $user_id ) . "' AND status = 'active' LIMIT 1";
		$row = $wpdb->get_row( $sqlQuery );
		
		// Now update it
		if ( ! empty( $row ) ) {
			$sqlQuery = "UPDATE $wpdb->pmpro_memberships_users SET billing_amount = '" . esc_sql( $billing_amount ) . "', cycle_number = '" . esc_sql( $cycle_number ) . "', cycle_period = '" . esc_sql( $cycle_period ) . "' WHERE id = " . esc_sql( $row->id ) . " LIMIT 1";
			$wpdb->query( $sqlQuery );
			echo "Found user with ID #" . $user_id . " and updated their active membership to use the billing values you set.";
		} else {
			echo "The user with ID #" . $user_id . " doesn't have an active membership.";
		}
		
		exit;
	}
}
add_action( 'init', 'my_fix_billing_amount' );