Open Database Diagrams (Visual Database Tools)
You can open database diagrams to view or edit the diagram's structure.
To open a database diagram
In Object Explorer, expand the Database Diagrams folder.
Double-click the name of the database diagram you want to open.
Right-click the name of the database diagram you want to open, and then choose Design Database Diagram.
The database diagram is opened in the Database Diagram Designer, from which you can edit the diagram.
Only the owner of the diagram or a member of the db_owner role of the database can open the diagram.