It's not possible to amend the leave date on a P45 which has been printed, however, an archived worker can be un-P45d in the same tax year during which they left.
Once you've printed a P45 the date may not be amended.
If they were made a leaver in the current tax year, you can unP45 them (previously known as cancel leaver). This renders the previous P45 null and void, allowing the worker to be left again with the correct date.