ECMA Script for UpdateList

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');
list.set_description('My custom generic list description');
list.update();
clientContext.load(list, 'Description');
clientContext.executeQueryAsync(Function.createDelegate(this, this.onSiteLoadSuccess), Function.createDelegate(this, this.onQueryFailed));
}
function onSiteLoadSuccess(sender, args) {
alert("list description : " + this.list.get_description());
}

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