Indexes can slow INSERT, UPDATE, and DELETE performance. If you find that an index hinders overall performance or you no longer need it, you can delete it.
To delete an index
Caution Choosing Delete will result in an action that cannot be undone without loosing 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 index is deleted from the database when you save your table or diagram.