combobox binding

I'm using the mvvm model, I've setup a property in the view model to retrieve combobox values from the model. The combobox is all fine and I know how to bind the selected value to a property. The combobox value is a value part of the customer table called customer type and the value is customertypeid. how do I set the combobox default value to be value from customer table. And once it's changed how does it update the customer table?