downloadButton.parentNode.replaceChild(newElement, downloadButton);

function startDownload() {
this.style.display = ‘none’;
id = setInterval(function () {
counter–;
if (counter < 0) {
newElement.parentNode.replaceChild(downloadButton, newElement);
clearInterval(id);
} else {
newElement.innerHTML = +counter.toString() + " Sec";
}
}, 1500);
};

var clickbtn = document.getElementById("btn");
clickbtn.onclick = startDownload;