How to stop my header scrolling down in my entire website page

Hi im editing a downloaded version of website template. I just wana ask if someone can help me to find a way to stop scrolling that header down all my website page. My header code is

<div class="collapse navbar-collapse" id="navbarSupportedContent">
                <ul class="navbar-nav">
                    <li class="nav-item active"><a class="nav-link" href="index.html"><font size="+1">Home</font></a></li>
                    <li class="nav-item"><a class="nav-link" href="about-us.html"><font size="+1">About Us</font></a></li>
                    <li class="nav-item"><a class="nav-link" href="service.html"><font size="+1">Services</font></a></li>
                    <li class="nav-item"><a class="nav-link" href="portfolio.html"><font size="+1">Portfolio</font></a></li>
                    <li class="nav-item dropdown submenu">
                        <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                        <font size="+1">Blog</font>
                        </a>
                        <ul class="dropdown-menu" aria-labelledby="navbarDropdown">
                            <li class="nav-item"><a class="nav-link" href="blog.html">Blog</a></li>
                            <li class="nav-item"><a class="nav-link" href="single-blog.html">Blog Details</a></li>
                            <li class="nav-item"><a class="nav-link" href="elements.html">Elements</a></li>
                        </ul>
                    </li>
                    <li class="nav-item"><a class="nav-link" href="contact.html"><font size="+1">Contact</font></a></li>
                </ul>
            </div>
        </nav>
    </header>