If you are the admin, you will have to send them back to the employees (by flagging the reports), who should then delete the created reports and re-create them based on your criteria.
Similarly, to modify approved reports, you can flag the reports - after which the employee can make modifications, add attachments etc.
As an employee, if your report is not approved, you can delete the created report and re-create them.
You will not be able to modify approved reports.
To edit an approved report, kindly contact your approver.