Change button responsive menu image?

Hi Triep. Try adding the following CSS to a child theme style.css file or a plugin like https://www.ads-software.com/plugins/simple-css/ or https://www.ads-software.com/plugins/simple-custom-css/:

/* add "Menu" to mobile menu icons */
.nav-toggle .fa-bars:before {
    content: "Menu \f0c9";
    display: block;
    width: 100px;
    float: right;
}