I'm in a rush to release so I am adding features that are needed to make it usable.

update_username.php

@@ -0,0 +1,53 @@
+<?php
+session_start();
+require_once 'includes/globals.php';
+require_once 'vendor/autoload.php';
+
+use DJMixHosting\Database;
+
+if (!isset($_SESSION['user'])) {
+    header("Location: login.php");
+    exit;
+}
+
+if ($_SERVER['REQUEST_METHOD'] !== 'POST' || !isset($_POST['new_username']) || empty($_POST['new_username'])) {
+    $_SESSION['error'] = "New username is required.";
+    header("Location: profile.php");
+    exit;
+}
+
+$new_username = trim($_POST['new_username']);
+
+// Validate username (for example, only alphanumeric and underscores, 3-25 characters)
+if (!preg_match('/^[a-zA-Z0-9_]{3,25}$/', $new_username)) {
+    $_SESSION['error'] = "Invalid username format.";
+    header("Location: profile.php");
+    exit;
+}
+
+$db = new Database($config);
+$userId = $_SESSION['user']['id'];
+
+// Check if the new username already exists (excluding the current user)
+$stmt = $db->prepare("SELECT id FROM users WHERE username = ? AND id != ?");
+$stmt->bind_param("si", $new_username, $userId);
+$stmt->execute();
+$result = $stmt->get_result();
+if ($result->num_rows > 0) {
+    $_SESSION['error'] = "Username already taken.";
+    header("Location: profile.php");
+    exit;
+}
+$stmt->close();
+
+// Update the username in the database
+$stmt = $db->prepare("UPDATE users SET username = ? WHERE id = ?");
+$stmt->bind_param("si", $new_username, $userId);
+$stmt->execute();
+$stmt->close();
+
+// Update session data
+$_SESSION['user']['username'] = $new_username;
+$_SESSION['success'] = "Username updated successfully.";
+header("Location: profile.php");
+exit;