Site/loginChange.php

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

$login = $_POST["login"];

$mysql = mysqli_connect($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 ($login))
if (isset ($_SESSION['id']))
$user_id = $_SESSION['id'];
$userlogin = $_SESSION['login'];
$quary = "SELECT * FROM Login WHERE login like'$login'";
$result = mysqli_query($mysql, $quary);
if(mysqli_num_rows($result) > 0 && $userlogin == $login)
{
    header('Location: LoginChange.html?message=Логины+совпадают');
    mysqli_close();
    session_abort();
}
if(mysqli_num_rows($result) > 0 && $userlogin != $login)
{
    header('Location: LoginChange.html?message=Логин+существует');
    mysqli_close();
    session_abort();
}
session_abort();
if($userlogin != $login && mysqli_num_rows($result) == 0)
{
$query = "UPDATE Login SET login = '$login' WHERE id_user = $user_id";
if($res = mysqli_query($mysql, $query))
{
    session_start();
    $_SESSION['login'] = $login;
    echo "<script>localStorage.setItem('login', '$login');
    window.location.href = 'UserAccount.html';</script>";     
}
}
?>