How can I modify a schema header

I am attempting to write an application using 5.1 and Parse.com.

When a user logs into the app, they are given a sessionToken. In order to use the REST API, I need to add that sessionToken to the model schema header. How can I modify the model schema header in run-time?

Currently my model schema looks like the code below. I have sessionToken as a global javascript var. Obviously that isn't working.

Code:

schema : {
    id : 'users' ,
    namespace : 'top.model' ,
    proxy : {
        type : 'rest' ,
        url : 'http://ift.tt/1nKE0Gh' ,
        useDefaultXhrHeader : false ,
        withCredentials : false ,
        headers : {
            'X-Parse-Application-Id' : "xxxxxxxx" ,
            'X-Parse-REST-API-Key' : 'xxxxxx' ,
            'X-Parse-Session-Token' : sessionToken,
            'Content-Type' : 'application/json'
 },
        reader : {
            type : 'json' ,
            rootProperty : 'results' ,
            transform: function ( data ) {
                return  data.results ? data.results : {results :data};
            }
        },
        writer : {
            type : 'json' ,
            writeRecordId : false
 }
    }
}

How can I modify a schema header