How do I update a database Diagram in Microsoft SQL Server Management Studio

It looks like the best way is to right click on the diagram and choose "remove table from diagram". Then right click in the white space and choose "Add Table". It will then show newly created relationships.

Close the diagram. Right click on the diagram name under the "Database Diagrams" node on the tree to the left and select "Refresh", then re-open the diagram. That worked for me when I was trying to get the new fields, updated field names, and updated "nullable" values to display. It also didn't scramble the custom layout I did in the diagram, which I was afraid might happen. If your diagram is missing entire tables, then open the diagram, right click in the diagram's blank space, and select "Add Table" to add missing tables.

This was the first result that came up when I was trying to figure out how to refresh the tables to reflect new fields and such. I was hoping I wouldn't have to delete/re-add each table one-by-one!