Reply To: Header Menu fitting

You could change the font size for the menu items.
18px is the standard size. Change it to a smaller value – 17px or 16px – and see if the full menu will then fit on one line.

.main-navigation a {
    font-size: 18px;
}

Or change the space between each menu item.
10px is standard. Change it to a lower value.

.main-navigation a {
    margin-left: 10px;
    margin-right: 10px;
}

To add this custom CSS code, go to Appearance > Customize > Additional CSS and write/copy it there.