TOP Navigation stays when page is scrolled

Question

I have a very minimal knowledge of CSS and am wondering if someone can help me with my current issue. I have an existing website with a fixed Top Navigation Menu (CSS &html codes for just the top navigation menu is shown below, no JavaScript or any script related to this).

I needed to make this top navigation stays when the page is scrolled up and down. Is this a matter of adding some JavaScript and or a few new CSS elements to the original CSS or I practically need to discard the existing CSS for this top navigation and create new CSS codes?

* {
  box-sizing: border-box;
}

body {
  font-family: 'Montserrat', sans-serif;
  line-height: 1.6;
  margin: 0;
  min-height: 100vh;
}

ul {
  margin: 0;
  padding: 0;
  list-style: none;
}

h2,
h3,
a {
  color: #34495e;
}

a {
  text-decoration: none;
}

.logo {
  margin: 0;
  font-size: 1.45em;
}

.main-nav {
  margin-top: 5px;
}

.logo a,
.main-nav a {
  padding: 10px 15px;
  text-transform: uppercase;
  text-align: center;
  display: block;
}

.main-nav a {
  color: #34495e;
  font-size: .99em;
}

.main-nav a:hover {
  color: #718daa;
}

.header {
  padding-top: .5em;
  padding-bottom: .5em;
  border: 1px solid #a2a2a2;
  background-color: #f4f4f4;
  -webkit-box-shadow: 0px 0px 14px 0px rgba(0, 0, 0, 0.75);
  -moz-box-shadow: 0px 0px 14px 0px rgba(0, 0, 0, 0.75);
  box-shadow: 0px 0px 14px 0px rgba(0, 0, 0, 0.75);
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  border-radius: 5px;
}


/* ================================= 
       Media Queries
       ==================================== */

@media (min-width: 769px) {
  .header,
  .main-nav {
    display: flex;
  }
  .header {
    flex-direction: column;
    align-items: center;
    .header {
      width: 80%;
      margin: 0 auto;
      max-width: 1150px;
    }
  }
}

@media (min-width: 1025px) {
  .header {
    flex-direction: row;
    justify-content: space-between;
  }
}

<header class="header">
  <h1 class="logo"><a href="#">LOGO</a></h1>
  <ul class="main-nav">
    <li><a href="#">Home</a></li>
    <li><a href="#">About</a></li>
    <li><a href="#">Portfolio</a></li>
    <li><a href="#">Contact</a></li>
  </ul>
</header>