You can do all operations in JavaScript itself.
Lets store the date coming from server into variable a:
var a = {
"XXXX":{"type":"RSS","value":"},
"YYYY (mins)":{"type":"String","value":""},
"ZZZZ":{"type":"String","value":""}
}
To change the value:
a['XXXX']['value'] = 'new_value1';
a['YYYY']['value'] = 'new_value2';
a['ZZZZ']['value'] = 'new_value3';
To add a field:
a["MMMM"] = {"type":"Image","value":"a7e8bec0-87ed-11e2-aa2e-52540025ab96_2_1362746556"}}