Delete a
To delete a primary key constraint
In your database diagram or Table Designer, select the primary key columns for the table whose primary key constraint you want to delete. A primary key column is identified by a primary key symbol in its row selector.
or
Caution Choosing Delete will result in an action that cannot be undone without losing all other changes made to the database diagram. To undo this action, close this database diagram and all other open database diagrams without saving the changes.
The constraint is deleted from the database when you save the table or diagram.
Constraints | Defining a Primary Key | Primary Key Constraints