If the ‘glyf’ table (that is, ‘glyf’ program) for a composite character like Å says “use my metrics,” you cannot use the Edit Metrics command (Ctrl + M) on the Edit menu to make changes. Instead, you must choose Glyph Program on the Views menu to display the ‘glyf’ table. There you can see which character the composite refers to. Then you can go to that character in the main window and edit its metrics.
If the composite doesn’t say “use my metrics,” you can use the Edit Metrics command—but only to edit the advance width. Visual TrueType calculates the left side-bearing based on the way it puts together the composite.