var UserSchema = new Schema({
  username: { type: String, required: true },
  password: { type: String, required: true },
  userType: { type: String, default: 'user'},
  quizzHistory: [{
    quizzName: String,
    quizzScore: Number
  }]
});

User.findOneAndUpdate({ _id: req.session.user['_id'], 'quizzHistory.quizzName': testName},
{
    '$set': {
        'quizzHistory.$.quizzName': testName,
        'quizzHistory.$.quizzScore': finalScore
    }
}, {upsert: true},
function(err, upd) {
  console.log("added");
})