It is always advised that you do not edit the theme directly. You should setup a child theme first. There are many good tutorials online for that.
In my CSS document I added this code:
.fontawesome-text {
display: block;
padding-top: 10px;
}
Then in the menu section of wordpress, for every menu item, fill-in the CSS Classes field with your desired fontawesome icon. One of mine is fa-car
. The CSS Classes field may not be there, so you have to turn it on by going up to the top of the page under Screen Options.