How to update model and view model in MVVM pattern?

I am struggling to implement MVVM pattern in my current project.

"ClassA" continuously gets required data from a remote device and stores this data inside it's fields. It is a model, I guess. ClassA updates required information via Update method.

"ClassB" continuously gets the data from "ClassA" and stores it in corresponding properties. Looks like it is a view model.

View is a simple MainWindow.xaml with a DataGrid inside.

I have the following questions:

1) How do I update ViewModel? Should ClassB have an Update method, which accepts an instance of ClassA and updates corresponding fields?

2) Where do I store an instance of ClassA? Should ClassA be a field of ClassB? If it should, then how do I update Model? I thought of something like the following:

public void UpdateB()
{
    ClassA.UpdateA();
    this.FieldOne = ClassA.FieldOne;
    this.FieldTwo = ClassA.FieldTwo;
}

4) Does model have it's update method at all or model just stores the data?

3) What do I do inside MainWindow.cs, aside from windows initialization? Do I update view model (ClassB) there?