It is possible to change the termination reason for a leaver after they have been terminated.
You will need the permission called 'Termination Editor' which can be added for you by your manager or any System Owner user by following the steps in our article on adding permissions.
To change the termination reason:
Search for the employee and go to their profile.
Go to Employment and click Position.
Click Edit Terminated Employee.
Change the option in the Reason Description dropdown.
Click Save.