The look of a WordPress menu can be changed by updating the CSS.

Note: Before making any changes to a WordPress theme, add a child theme

Log in to your WordPress admin area.

Go to Appearance > Editor.

Enter the desired changes to the CSS.

Example: This is sample code for changing the font size for a WordPress menu:

                .primary-navigation {
                 font-size: XXpx;
               }

The XX is the new font size for your menu. The px stands for pixel.

Click Update File.