<script language="ecmascript" type="text/ecmascript">
 
    var clientContext = null;
    var oWeb = null;
    var oListColl = null;
    var oList = null;
    var oListItem = null;
    
   
 
    function updateListItem() {
        clientContext = new SP.ClientContext.get_current();
        oWeb = clientContext.get_web();
        oListColl = oWeb.get_lists();
        oList = oListColl.getByTitle('Custom List');      
        oListItem = oList.getItemById(1);
        oListItem.set_item('Title', 'Item Updated');
        oListItem.update();        
        clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
    }
 
    function onQuerySucceeded() {
 
        alert('item is updated successfully.');
    }
 
    function onQueryFailed(sender, args) {
        alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
    }
 
 
 
</script>
<input id="btnUpdateListItem" onclick="updateListItem()" type="button" value="Update List Item"/>