When you "update a model from a database", you are making sure the model reflects the same state, that is, which tables exist in the database with what columns and so on, as the database.
With one of more database sessions, start the Diagram Editor.
Next, load the diagram you want to update from the database.
The Tools button
From the Tools tool bar button, select Update from Database.
If you have multiple session open, the Select Session dialog will open.
The Select Session dialog
The dialog allows you to select a single session to attach to.
The lists of Tables, Views and Domains in the Diagram Explorer will be updated with the objects from the database so that they can be used in the diagram.
Part of the Diagram Explorer
You can drag and use items from the Diagram Explorer onto the Canvas and use them.
See also: Reverse Engineering