It depends on the status of the eGC1. You can delete an attachment from an eGC1 if the eGC1 is still in composing status, or if the eGC1 has been completed but is withdrawn before anyone has approved it. After an eGC1 has been approved, the attachments can be updated, but not deleted.
Note: Grant Runner allows you to delete files when the eGC1 is in Withdrawn or Returned status if the file is not a required one.
If you have attached a file to the wrong place in an eGC1, you can update it with a document that indicates the file is incorrect and should be ignored.