如何在Jquery表单验证插件中更改"此字段是否必需"的内容？

<form id="myForm">
 <label for="username">用户名：</label>
 <input type="text" id="username" name="username" required>
 <button type="submit">提交</button>
</form>

$("#myForm").validate({
  messages: {
    username: {
      required: "请输入用户名"
    }
  }
});

$("#myForm").validate({
  messages: {
    username: {
      required: "请输入用户名"
    },
    password: {
      required: "请输入密码"
    }
  }
});