Well, the issue is you have too many items in your top-level menu. I could reduce the font size a bit, but that still doesn’t help when the browser is a bit narrower.
So, I recommend reorganizing your menu after applying this CSS to reduce inter-item spacing:
nav li a {
padding-left: .75em;
padding-right: .75em;
}
To add or override CSS: use the “Additional CSS” option in the customizer.? https://codex.www.ads-software.com/CSS#Custom_CSS_in_WordPress
Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.