Site/passwordChange.php

<?php
$host = "provalgk.beget.tech";
$username = "provalgk_user";
$password = "Qwer123.";
$database = "provalgk_user";

$pass = $_POST["password"];

$mysql = mysqli_connect($host, $username, $password, $database);

// Проверка соединения
if($mysql->connect_error){
    die("Ошибка подключения". $mysql->connect_error);
}

// Получение данных пользователя из базы данных
session_start();
if (isset ($pass))
if (isset ($_SESSION['id']))
$user_id = $_SESSION['id'];
$userpass = $_SESSION['password'];
if($userpass == $pass)
{
    header('Location: PasswordChange.html?message=Пароли+совпадают');
    echo "<script>window.location.href = 'PasswordChange.html';</script>";
}
else{
$query = "UPDATE Login SET password = '$pass' WHERE id_user = $user_id";

if($res = mysqli_query($mysql, $query))
{
    $_SESSION['password'] = $pass;
    echo "<script>window.location.href = 'UserAccount.html';</script>";   
}
}
?>