Edit in JSFiddle

var btn = document.getElementById("btn");
var flag = false;
btn.onclick = function() {
    flag = !flag;
    if (flag) {
        var str = document.getElementById("url").innerHTML;
        var re_str = str.replace("-s", "-l");
        document.getElementById("url").innerHTML = re_str;
    } else {
        var str = document.getElementById("url").innerHTML;
        var re_str = str.replace("-l", "-s");
        document.getElementById("url").innerHTML = re_str;
    }
};

<div id="wrapper">
<h2 id="url">image/test-s.jpg</h2>
<button id="btn">imagechange</button>
</div>

div#wrapper{
    width:200px;
    margin:60px auto;
}

h2{
    font-size:20px;
    font-weight:bold;
    margin-bottom:20px;
}