Delete all post_meta.

meta-reset.php

<?php
class Meta_Reset {
	function __construct() {
		add_action( 'parse_request', [ $this, 'parse_request' ] );
	}

	/**
	 * Usage: mydomain.com/?meta_reset=1
	 */
	function parse_request() {
		global $wpdb;

		$table = _get_meta_table( 'post' );
		$meta_keys = [
			'_my_meta_key',
		];

		if ( isset( $_GET[ 'meta_reset' ] ) ) {
			foreach ( $meta_keys as $meta_key ) {
				$query = "DELETE FROM $table WHERE meta_key = '$meta_key'";
				$wpdb->query( $query );
			}
		}
	}
}