$args = array(
                'number'     => -1,
                'fields'     => 'ids',
                'meta_query' => array(
                    array(
                        'key'     => 'wpf_complete',
                        'compare' => 'EXISTS',
                    ),
                ),
            );

            $payments = edd_get_payments( $args );

            foreach ( $payments as $payment_id ) {
                delete_post_meta( $payment_id, 'wpf_complete' );
            }