[Theme: Riba Lite] How to hide mobile menu?

Hi

How do I hide/disable the original menu from my website?Currently it shows both WP responsive menu and the original theme menu.

I have installed custom css pluging and failed even after adding this code:
@media screen and (max-width: 650px) {

.js .main-nav {
height: auto;
}

.js .main-nav .menu {
display: block;
}

.js .main-nav .menu {
top: 0;
position: relative;
}

}
}