You can create app.yml manually (there is a template in /var/discourse/samples/standalone.yml) – but if you already bootstrapped and want to downgrade, this is easy as long as you are willing to lose your data. Simply update app.yml, then run this:
cd /var/discourse/
./launcher stop app
rm -rf shared/standalone/ # wipe shared volume
./launcher rebuild app
Executing these commands will completely reset Discourse! All content, users and settings will be deleted.