Update Json data.

Hi All,
I am trying following code for update JSON data using jquery.
This code executing without any error but value is not getting updated in JSON.
Can anyone please tell me how to update data ? Is I am missing anything ?
Code:
    var json_string = $('input[id$=_hfValidation]').val();     var objData = jQuery.parseJSON(json_string);     // parse it     objData.ChannelWeightProposed = this.value;  // change a value     alert('objData.ChannelWeightProposed  ' + objData.ChannelWeightProposed);     //json_string = $.toJSON(objData);            // resave it     json_string = JSON.stringify(objData);
Thanks,
Harish Patil