If an expense has the status 'reported', it is no longer possible to make any changes to the details. If any changes are needed, you firstly need to unreport the expense manually.
If you open the details of the relevant expense, you can click on the button 'Un-Report' to change the expense status back to the previous one:
You can also unreport multiple expenses at once by selecting all expenses you want to unreport and clicking on the 'Actions'-button for the same functionality:
As a note:
Be careful with this feature! By manually unreporting an expense, it will still be available in the created accounting report that changed its status to reported. Set it back as reported afterwards or delete the accounting report as well.