Hello,

Please add this code to the Theme Settings > Custom CSS:

.basel-navigation .menu>li.menu-item-design-default ul li a,
.main-nav .menu>li>a {
    text-transform: lowercase;
}

If you want the menu to have the first capital letter use this code instead:

.basel-navigation .menu>li.menu-item-design-default ul li a,
.main-nav .menu>li>a {
    text-transform: capitalize;
}

Best Regards