How to: Delete Primary Keys (Visual Database Tools)
Delete a primary key constraint when you want to remove the requirement for uniqueness for the values entered in a column or a combination or columns.
To delete a primary key constraint
In Object Explorer, right-click the table with the primary key, and click Design (Modify in SP1 or earlier).
The table opens in Table Designer.
In the table grid, right-click the row with the primary key and choose Remove Primary Key to toggle the setting from on to off.
To undo this action, close the table without saving the changes. Deleting a primary key cannot be undone without losing all other changes made to the table.
The constraint is deleted from the database when you save the table or diagram.