For more details, see:
http://www.obofoundry.org/ontology/exo.html
Only ever edit the file src/ontology/exo-edit.obo (using Protege), not exo.obo or exo.owl.
Follow this advice for creating new terms:
https://oboacademy.github.io/obook/howto/create-new-term/
cd src/ontology
sh run.sh make prepare_release