DAO design to update an entity in DB

Lets say a client updates an entity ( Ex: student entity ). So we get the student Id and other modified fields (not all fields) from the client.

I read that we should pass the particular entity object to DAO in order to update. But then , how will I get to form that entity object.Because I don't have all fields data to create a proper entity object.

Should I make two DB calls ?

The first call is to construct a proper entity object and then make the update by passing this updated entity object to the DAO.