You can use ckeditor or markitup, settings for the editor are in fuel/application/config/MY_fuel.php
If you want to change core editor files (I've updated ckeditor versions for a few projects without any problems), they are in fuel/modules/fuel/assets/js/editors/
Be careful though, as they are in the fuel module folder they may be overwritten if you update fuelcms in the future.
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If you want to change core editor files (I've updated ckeditor versions for a few projects without any problems), they are in fuel/modules/fuel/assets/js/editors/
Be careful though, as they are in the fuel module folder they may be overwritten if you update fuelcms in the future.