How can I change the font of my primary menu

Find this:

.ht-main-navigation a {
    color: #444;
    display: block;
    font-family: "Oswald",sans-serif;
    font-size: 14px;
    font-weight: 400;
    line-height: 36px;
    padding: 0 15px;
    text-decoration: none;
    text-transform: uppercase;
    transition: none 0s ease 0s ;
}

and replace font-family: line with
font-family: "Arial",sans-serif;

Adding on what sninip posted – here are 2 resources.

You can learn about CSS Fonts here: https://www.w3schools.com/css/css_font.asp

You can learn about CSS Text here: https://www.w3schools.com/css/css_text.asp