Always backup your volumes. You can locate your volumes on disk with docker volume list and docker volume inspect to identify the "Mountpoint" with data. Graylog upgrades can be complicated based on changes to Elasticsearch and Mongodb. In some cases, it's enough to just update the version of graylog in your compose file as long as that version is compatible with exiting versions of Mongo and ES. In some cases you may have to follow external procedures to upgrade all components. Since I wrote this, I've updated from 4.1 to 4.3. The only change I made was the version number in my compose file and restart.