Add or change a value of JSON key with jQuery or JavaScript

I have a JSON string(?) that I have returned from $.ajax() and named it data. Some of the values are empty and I need to add values to some of the keys and send it back to my PHP script.

I access the existing values by data.keyName. How do I add or change the values of certain keys in data?

This is what data looks like.

{
    "ID":"48",
    "userID":"0",
    "address":"750 North High Street",
    "city":"Columbus",
    "state":"OH",
    "zip":"43215",
    "lat":"39.977673",
    "lng":"-83.003357",
    "busNumber":"55",
    "isClaimed":"N",
    "whereFound":"",
    "busNum":"",
    "email":"",
    "fname":"",
    "lname":"",
    "comments":""
}