How to manipulate mongoose query result in exec callback

I am a newer in meanjs. Now I am stuck in how to manipulate the data queried from mongodb using mongoose. Here is my case:

Model schema is this:

var AdminUserSchema = new Schema({
firstName: {
    type: String,
    trim: true,
    default: '',
    validate: [validateLocalStrategyProperty, 'Please fill in your first name']
},
lastName: {
    type: String,
    trim: true,
    default: '',
    validate: [validateLocalStrategyProperty, 'Please fill in your last name']
},
displayName: {
    type: String,
    trim: true
},
email: {
    type: String,
    trim: true,
    default: '',
    validate: [validateLocalStrategyProperty, 'Please fill in your email'],
    match: [/.+\@.+\..+/, 'Please fill a valid email address']
},
username: {
    type: String,
    unique: 'Username already exists',
    required: 'Please fill in a username',
    trim: true
},
password: {
    type: String,
    default: '',
    validate: [validateLocalStrategyPassword, 'Password should be longer']
},
salt: {
    type: String
},
provider: {
    type: String,
    required: 'Provider is required'
},
providerData: {},
additionalProvidersData: {},
roles: {
    type: [{
        type: String,
        enum: ['user', 'admin']
    }],
    default: ['admin']
},
updated: {
    type: Date
},
created: {
    type: Date,
    default: Date.now
},
/* For reset password */
resetPasswordToken: {
    type: String
},
resetPasswordExpires: {
    type: Date
},
enabled: {
    type: Boolean,
    default: true
}

});

When I try to get data and manipulate with following code:

var AdminUser = mongoose.model('AdminUser');
AdminUser.find().exec(function(err, users) {
    if(err) {
        return res.status(400).send({
            message: errorHandler.getErrorMessage(err)
        });
    } else {

            users[1].password = undefined;
            users[1].salt = undefined;
            var created = moment(users[1].created).format('YYYY-MM-DD');
            users[1].created = created;

            console.log(created);
            console.log(users[1].created);
            console.log(users[1]);
    }
});

The result shows as follow, it just doesn't work.

2015-04-07

Tue Apr 07 2015 12:00:00 GMT+1200 (NZST)

{ _id: 55233e0c488bd50b1eafe558, salt: undefined, displayName: 'test test', provider: 'local', username: 'test', __v: 0, enabled: true, created: Tue Apr 07 2015 12:00:00 GMT+1200 (NZST), roles: [ 'admin' ], password: undefined, email: 'test@test.com', lastName: 'test', firstName: 'test' }

I have tried to manipulate with like this: users[1]['created'] = created;

or using set function: users[1].set('created', created);

both failed. Please give a hand.

Any help would be appreciated.