ECMA Scritp for UpdateListItem

var clientContext = null;
var web = null;
ExecuteOrDelayUntilScriptLoaded(Initialize, "sp.js");
function Initialize()
{
clientContext = new SP.ClientContext.get_current();
web = clientContext.get_web();
this.list = web.get_lists().getByTitle('My custom generic list');
this.oListItem = list.getItemById(1);
oListItem.set_item('Title', 'Abhijeet Updated');
oListItem.update();

clientContext.executeQueryAsync(Function.createDelegate(this, this.onUpdateListItemSuccess), Function.createDelegate(this, this.onQueryFailed));
}
function onUpdateListItemSuccess(sender, args) {
alert("list item updated");
}

function onQueryFailed(sender, args) {
alert('request failed ' + args.get_message() + '\n' + args.get_stackTrace());
}