Cant get Test Data Builder example to work

I'm trying to get a simple example of Test Data Builder Pattern to work. But I can't access the .build method from my Builderclass.

User user = UserBuilder().buildDefaultUser();

I have created a simple Model User class:

public class User
{
     private String firstName;
     private String surname;
     private int age;
}

public void setFirstName(String firstName)
{
     firstName = firstName;
}

//repeating for all properties

I have then created a Builder Class:

public class UserBuilder
{

     public User buildDefaultUser()
     {
         User user = new User();
         user.setFirstName("Tom");
         user.setSurname("Jones");
         user.setAge(29);

         return user;
     }  
 }

When I try to set specific values for the user, rather than using default value by doing:

public class Test1
{

      public void DataSetup()
      {
          User user = UserBuilder().buildDefaultUser();
          user.setSurname("Fred");
      }
}

I am not able to and get 'An object reference is required for the non-static field, method, or property'

Any help appreciated.
Thanks Ami.