The system does not allow you to edit the employee's final date of employment if it's in the past. In this case, you will need to rehire the employee with the entitlements they had initially before leaving and input the correct leaver date when marking them as a leaver again.
Click Access Button
then click Employee.
Click the relevant employee then, under Quick Actions, click Leaver Wizard.
Click Remove Leaver.
Once you've done this, you can process the employee as a leaver again with the corrected details.