var downloadButton = document.getElementById("download");
var counter = 10;
var newElement = document.createElement("div");
newElement.innerHTML = "You can download the file in 10 seconds.";
var id;

newElement.classList.add('Class');

downloadButton.parentNode.replaceChild(newElement, downloadButton);

id = setInterval(function() {
    counter--;
    if(counter < 0) {
        newElement.parentNode.replaceChild(downloadButton, newElement);
        clearInterval(id);
    } else {
        newElement.innerHTML = "You can download the file in " + counter.toString() + " seconds.";
    }
},  1000);