Here's an unordered (<ul>) list of links:
Try this: add a class="vertical-navbar" attribute to this <ul>, then reload.
Try this: change to class="horizontal-navbar" for this <ul>, then reload.
By the way: in a normal situation, you would just give the <ul> a class like class="navbar", and then write the .navbar styling in the CSS file, and you wouldn't have to edit the HTML at all to modify the navbar appearance.
Ideally, we store content/meaning in the HTML file, and we determine the presentation/appearance in the CSS file.
I got the idea for this example from this tutorial on navigation bar styling