In order to edit the data in a report, it must be set up as a Formula rather than just a data type. To edit an existing row, replace it with the same data using the Formula editor. Read more about this here How to create or edit a formula in a report
Why can't I edit my row or change its time period in a report?
Why some report rows are locked and how to make a row editable by converting it to a formula row so you can change its time period.