If you do not have any custom question modules, an upgrade to 5.x is easy.
- Backup your database
- Delete the old Quiz module folder
- Copy the new Quiz module folder into place
- Run update.php
- Verify that existing quizzes work
- Verify that existing results work
- Done!
If you encounter any errors during update, please post them to https://www.drupal.org/project/issues/quiz