By "start all over" I assume you mean with a fresh install without repeating all the steps.
https://openacs.org/doc/current/snapshot-backup.html shows how to backup and recover.
Essentially, you can skip the backup, though it doesn't hurt to practice. Stop aolserver. For wiping out the old data (section 3), for postgresql, "dropdb $OPENACS_SERVICE_NAME". If you have modified the files, you will want to restore the original openacs tar files as well.
In the Recover section 4, there is no need to import the database after "createdb -E UNICODE $OPENACS_SERVICE_NAME". Just restart aolserver and browse to the front page to re-install a new OpenACS.
