How to Update post_modified of all wordpress post

i want to update all my wordpress post modified date and time to a specific date and time,
i have a code to update it base on the id, but i want one to update all the row at once without putting the id of the posts .

if ($conn->query($sql) === TRUE) {
  echo "Record updated successfully";
} else {
  echo "Error updating record: " . $conn->error;
}

$conn->close();