const firestore = firebase.firestore();
// get the data from 'name@xxx.com'
firestore.collection("users").doc("name@xxx.com").get().then(function (doc) {
    if (doc && doc.exists) {
        var data = doc.data();
        // saves the data to 'name'
        firestore.collection("users").doc("name").set(data).then({
            // deletes the old document
            firestore.collection("users").doc("name@xxx.com").delete();
        });
    }
});