Add this to your css –
.nav-primary .genesis-nav-menu {
text-align: center;
}
.nav-primary .genesis-nav-menu .menu-item {
display: inline-block;
float: none;
}
Make sure and add it in the main section and not in the responsive section of your css so it will work when viewed on desktop/laptops. It already works on mobile and tablet devices since it’s designed to center on smaller resolutions.