How to: Delete Unique Constraints (Visual Database Tools)
Delete a unique constraint when you want to remove the requirement for uniqueness for values entered in the column or combination of columns included in the constraint expression.
In Object Explorer, right-click the table with the unique constraint, and click Design (Modify in SP1 or earlier). The table opens in Table Designer.
From the Table Designer menu, click Indexes/Keys.
In the Indexes/Keys dialog box, select the unique key in the Selected Primary/Unique Key and Index list.
The constraint is removed from the database when you save the table.