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
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Open the Table Designer for the table whose index you want to delete, right-click in the Table Designer, and choose Properties from the shortcut menu.
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 and Table Designer windows without saving the changes.
The index is deleted from the database when you save your table or diagram.
Creating an Index | Indexes | Saving, Displaying, and Updating an Index Definition