更新JSON值

I'm having a problem with updating json value.
This is how I wan't to do it (results.rows.item(0) - SQLite query result, 1 row only):

data[tableName]=[results.rows.item(0)];

var signatureArray=data[tableName][0].signature.match(/.{1,6000}/g);

data[tableName][0].signature = '';

This is what I get then from console.debug(data):

Object {registers: Array[1]}
    registers: Array[1]
        0: Object
            comments: "asd adas dasa"
            id: 2
            modules: "1, 0"
            passfail: 1
            present: 1
            signature: "iVBORw0KGgoAAAANSUhEUgAAAr8AAADICAYAAADhoYm+AAAYRElEQ..."

I'm using tableName because this is dynamic value. I'm taking values from many tables.

I want to remove signature value/set to '', because I need to send this data to remove server using ajax, jsonp, and with signature url is too long (414 error). I can't use POST.

also when i do this:

var signatureArray=data[tableName][0].signature.match(/.{1,6000}/g);

signatureArray is array containing 2 elements, so I get access to this value, but I can't change it.

How do I change data[tableName][0].signature???