Save Database Diagrams (Visual Database Tools)
Applies to: 
SQL Server
By saving a database diagram, you can save all the changes you made to it, including any changes you made to the tables, columns, and other database objects.
To save the database diagram
From the File menu, choose Save <tablename>.
If this is a new diagram that you have never before saved, the Save As dialog box appears. Type a name for the diagram.
If you made changes to tables in an existing diagram, the Save dialog box appears and displays a list of changes that will be saved in the database when you save your diagram.
Click Yes (or OK if this is a new diagram) to update the database to match your diagram.