You can use following;
var pageItems = '{"stationID":"145","categoryID":"-1","pickupDate":"2014-04-22","pickupTime":"08:00","returnDate":"2014-04-23","returnTime":"08:00","milage":"false","miles":"","attributes":[[],[]],"additionals":[[],[]]}';
var json = JSON.parse(pageItems);
json.categoryID = 65;
pageItems_v2 = JSON.stringify(json);
Here is a working demo : Demo