I can't update a value in my MongoDB from PHP

Question

I'm trying to do a php to update an user account, that I send from android:

$entityBody = file_get_contents('php://input');
$body = json_decode($entityBody,true);
foreach ($body as $value) 
{
    $username    = $value['username'];
    $oldPassword = $value['oldPassword'];
    $newPassword = $value['newPassword'];
}

I have 3 variables, the username of the account that I want to modify the password, the old password, and the new password.

I do the connection with my database.

$con = new MongoDB\Client;
if($con){
    $db = $con->users;
    // Select Collection
    $collection = $db->user;

Here, I make a filter to search the document that I want.

    $filter = array(['Username' => $username,
                     'Password' => $oldPassword
                     ]);

    $qry = new MongoDB\Driver\Query($filter);

I am using the username to find the user in the MongoBD.

    $rows = $collection->findOne($qry);

When I've found it, I use and if to see if the username and the password are correct.If it's, I'm trying to change the old password and put the new one,but it's not working.Putting var_dumps, I realized the mistake is in the update line, but I don't know what I am doing wrong in it.

    if($rows['Username']== $username && $rows['Password'] == $oldPassword)
    {
        $criteria = ['Username' => $rows['Username']];
        $newData = ['$set' => ['Password' => $newPassword]];

        $collection -> update($criteria,$newData);
        $changed = true;

         echo json_encode(array('status'=> '1','isChanged' => $changed));
    }
    else{

    $changed = false;
    echo json_encode(array('status'=> '2','isChanged' => $changed));
    }
    }

Finally, if the $con is false, I show an error message.

    else
    {
        die("Mongo DB not connected!");
    }