Get password hash to work on a password reset script

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SQL

PHP

I am trying to hash the password from the password reset scripts but it does not hash so if you can tell me where i am supposed to put it on the script. thank you very much

<?php
session_start();

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "vuyanii";

$conn = null;
try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    
    }
catch(PDOException $e)
    {
    echo $e->getMessage();
    }
    
    if(isset($_POST['submit'])){
    $username           = strip_tags($_POST['username']);
    $npassword      = ($_POST['password']); 
    $cpassword  = ($_POST['cpassword']);    
    //echo $username;exit();
    $message ="";

        try{
    $query = "SELECT * FROM users WHERE username= '".$username."'";
    $stmt = $conn->prepare($query);
    $stmt->execute();
    if($stmt->rowCount() < 1){
    $message = '<p style="color:red;">Incorrect username entered</p>';
    }else if($npassword != $cpassword){
        $message = '<p style="color:red;">New password does not match confirm password</p>';
        
    }
    else{
        $password = password_hash($npassword, PASSWORD_DEFAULT);
        $query = "UPDATE users SET 
        password        =  '$npassword' 
        WHERE username  =  '$username'";
        $stmt2 = $conn->prepare($query);

        if ($stmt2->execute()){

            $message = '<p style="color:red;">Password Changed Successfully</p>';
            header('location:index.php');
            
        }else{
            $message = '<p style="color:red;">Password Not Changed</p>';
            
        }
    }
    }catch(PDOException $e){ echo $e->getMessage();}
    }

?>

What I have tried:

tutorials and edited my script a couple of times