When you received a report for approval, but certain expenses in this report cannot be approved (e.d. due to the company's expense policy), you have the opportunity to modify an expense within the report, before approving it.
To modify an expense, select the expense that is to be corrected. A window with the expense details will open, and you can make the desired changes and save it again.
The history of a report will show the changes that were made and by who.
After this you can approve the report as you are used to.