How to access object property from inside class function

one of my classes in Javascript needs to be "updated" with Json sometimes. I have always done a function, that updates the array of data, given an id, but now i wanted to do it more encapsulated (Function update, inside the class).

What i made:

function File(data){
        this.data = data;

        this.update = function (callback){
            var set = function(ajaxData){
                this.data = ajaxData.PcbFile;
            }
            getPcbFile(data.id, function(ajaxData){
                set(ajaxData);
                callback();
            });
        };
    }

But, this.data = ajaxData.PcbFile; doesen't work... My object still with the last data set, and not the updated one. The function SET, i created as another attempt to set the data.

There is no problem on the ajax, since i debuged the ajaxData, and it's ok (when i update).

So, how do i really access the object property data from an inside function?

(sorry for my english...)