如何修改mongoose中已有的模式？

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true });

const userSchema = new mongoose.Schema({
  name: String,
  age: Number,
  email: String
});

const User = mongoose.model('User', userSchema);

userSchema.add({
  address: String
});

userSchema.path('email').options.type = Array;

const newUser = new User({
  name: 'John Doe',
  age: 25,
  email: ['john@example.com'],
  address: '123 Main St'
});

newUser.save((err) => {
  if (err) {
    console.error(err);
  } else {
    console.log('User saved successfully.');
  }
});