<script language="javascript" type="text/javascript">
_spBodyOnLoadFunctionNames.push("getSetListItem");
        var listItem;
        var list;
        var clientContext;
        function getSetListItem() {
            this.clientContext = SP.ClientContext.get_current();
            if (this.clientContext != undefined && clientContext != null) {
                var webSite = clientContext.get_web();
                this.list = webSite.get_lists().getByTitle("TextTestList");
                this.listItem = list.getItemById(2);
                clientContext.load(this.listItem);
                this.clientContext.executeQueryAsync(Function.createDelegate(this, this.OnLoadSuccess),
Function.createDelegate(this, this.OnLoadFailed));
            }
        }
        function OnLoadSuccess(sender, args) {
            var value = this.listItem.get_item("Country");
            this.listItem.set_item("Country", "US");
            this.listItem.update();
            this.clientContext.load(this.listItem);
            this.clientContext.executeQueryAsync(Function.createDelegate(this, this.OnLoadSuccess1),
Function.createDelegate(this, this.OnLoadFailed));
        }
        function OnLoadSuccess1(sender, args) {
            alert(this.listItem.get_item("Country"));
        }
        function OnLoadFailed(sender, args) {
            alert('Request failed. ' + args.get_message() + 'n' + args.get_stackTrace());
        }

        </script>

this.listItem = list.getItemById(2);