Yeah… well… no offense but you’ll have to dig yourself through the GitHub - zammad/zammad-docker-compose: Zammad Docker images for docker-compose Repo.
The compose has been adjusted and changed so much that there’s no guide or complete how to. I’d say backup, restore via the numerous stable mayor versions you’re missing out would be the best approach. 2.9 is from 2019… that is… old.
Also make sure to read all release notes during your upgrade before updating as you may encounter breaking changes and very important changes you should be aware of before.