script.js

let productImg = document.getElementById("productImg");
let productImg2 = document.getElementById("productImg2");
let btn = document.getElementsByClassName("btn");

btn[0].onclick = function () {
    productImg.src = "images/image1.png";
    productImg2.src = "images/image1.png";
    for (bt of btn) {
        bt.classList.remove("active");
    }
    this.classList.add("active");
}
btn[1].onclick = function () {
    productImg.src = "images/image2.png";
    productImg2.src = "images/image2.png";
    for (bt of btn) {
        bt.classList.remove("active");
    }
    this.classList.add("active");
}
btn[2].onclick = function () {
    productImg.src = "images/image3.png";
    productImg2.src = "images/image3.png";
    for (bt of btn) {
        bt.classList.remove("active");
    }
    this.classList.add("active");
}