Welcome to the QuickBooks Community! It would be a pleasure to shed some light on this situation here. Since the time has already been exported, QuickBooks puts a lock on that timesheet. In order to unlock it, you would have to contact us, and from there, we will guide you on how to undo those times in the approvals. You can call us at 888-836-2720 or chat with an expert by selecting this link.
Another workaround would be to adjust the time-off balance for that particular employee. I have added this video presentation that shows exactly how to do so.
Please let me know if you have any questions or need further clarification. I'm just a reply away, so don't hesitate at all! I hope you have a fantastic day.