HTML/CSS: How to move my navigation bar to the middle?

Question

I am new to CSS and I am making a navigation bar. Currently, my navigation bar is situated on the left and I would like to move the whole bar to the middle. How can I do that?

Below are the codes. Thanks in advance!

/* Navigation bar */
#navigation_bar {
    list-style-type: none;
    margin: 0;
    position: fixed;
    background-color: #333;
    top: 0;
    Left: 0;
    width: 100%;
}

li {
    float: left;
}

li a, .dropbtn {
    display: inline-block;
    color: white;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
}

li a:hover:not(.active), .dropdown:hover .dropbtn {
    background-color: #111;
}

li.dropdown {
    display: inline-block;
}

.dropdown-content {
    display: none;
    position: absolute;
    background-color: #f1f1f1;
    min-width: 160px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
}

.dropdown-content a {
    color: black;
    padding: 12px 16px;
    text-decoration: none;
    display: block;
    text-align: left;
}

.dropdown-content a:hover {
    background-color: #f1f1f1
}

.dropdown:hover .dropdown-content {
    display: block;
}

.active {
    background-color: #4CAF50;
}

<!--Navigation bar-->
        <ul id="navigation_bar">
            <li><a class="active" href="navigation_bar/home.html">Home.</a></li>
            <li><a href="navigation_bar/promotion.html">Promotion.</a></li>
<!--drop down menu-->
           <li class="dropdown"><a href="navigation_bar/hot_products.html" class="dropbtn">Hot Products.</a>
                <div class="dropdown-content">
                    <a herf="navigation_bar/sub_menu/sandwiches.html">Sandwiches</a>
                    <a herf="navigation_bar/sub_menu/burger.html">Burger</a>
                    <a herf="navigation_bar/sub_menu/rice.html">Rice</a>
                    <a herf="navigation_bar/sub_menu/noodles.html">Noddles</a>
                </div>
            </li>   
<!--Back to normal-->
            <li><a href="navigation_bar/cold_products.html">Cold Products.</a></li>
            <li><a href="navigation_bar/snacks.html">Snacks.</a></li>
            <li><a href="navigation_bar/about_us.html">About Us.</a></li>
            <li><a href="navigation_bar/contact_us.html">Contact Us.</a></li>
        </ul>

Answer 1

Try this

#navigation_bar{ text-align: center;}   /*add align center  */
li {display: inline; /* float:left; */} /* remove float left here and add display inline */

Answer 2

Update your #navigation_bar with

#navigation_bar {
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 0;
}

Here is the working Demo

#navigation_bar {
    list-style-type: none;
    margin: 0;
    position: fixed;
    background-color: #333;
    top: 0;
    Left: 0;
    width: 100%;
		display: flex;
    align-items: center;
    justify-content: center;
    padding: 0;
}

li {
    float: left;
}

li a, .dropbtn {
    display: inline-block;
    color: white;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
}

li a:hover:not(.active), .dropdown:hover .dropbtn {
    background-color: #111;
}

li.dropdown {
    display: inline-block;
}

.dropdown-content {
    display: none;
    position: absolute;
    background-color: #f1f1f1;
    min-width: 160px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
}

.dropdown-content a {
    color: black;
    padding: 12px 16px;
    text-decoration: none;
    display: block;
    text-align: left;
}

.dropdown-content a:hover {
    background-color: #f1f1f1
}

.dropdown:hover .dropdown-content {
    display: block;
}

.active {
    background-color: #4CAF50;
}

      <ul id="navigation_bar">
            <li><a class="active" href="navigation_bar/home.html">Home.</a></li>
            <li><a href="navigation_bar/promotion.html">Promotion.</a></li>
<!--drop down menu-->
           <li class="dropdown"><a href="navigation_bar/hot_products.html" class="dropbtn">Hot Products.</a>
                <div class="dropdown-content">
                    <a herf="navigation_bar/sub_menu/sandwiches.html">Sandwiches</a>
                    <a herf="navigation_bar/sub_menu/burger.html">Burger</a>
                    <a herf="navigation_bar/sub_menu/rice.html">Rice</a>
                    <a herf="navigation_bar/sub_menu/noodles.html">Noddles</a>
                </div>
            </li>   
<!--Back to normal-->
            <li><a href="navigation_bar/cold_products.html">Cold Products.</a></li>
            <li><a href="navigation_bar/snacks.html">Snacks.</a></li>
            <li><a href="navigation_bar/about_us.html">About Us.</a></li>
            <li><a href="navigation_bar/contact_us.html">Contact Us.</a></li>
        </ul>

Answer 3

Change navigation_bar css to :

#navigation_bar { 
list-style-type: none;
margin: 0 auto;
position: fixed;
background-color: #333;
top: 0;   
width: auto;    
display: block;
float: left;
}

Answer 4

By changing a bit of your CSS code you can easily do that as:

#navigation_bar {
   text-align: center;
}

#navigation_bar li {
    float: none;
    display: inline-block;
    vertical-align: top;
}
li .dropdown-content a:hover:not(.active) {
    background: #d0d0d0; /* for altering the hover effect on submenus */
}

I have also created a JSFiddle.