1. Fetch the latest changes from the *fdroiddata* repository.
2. Switch to a new branch in your copy of the *fdroiddata* repository.
3. Edit `metadata/com.fsck.k9.yml` to create a new entry for the version you want to release. Usually it's copy & paste of the previous entry and adjusting `versionName`, `versionCode`, and `commit` (use the tag name). Leave `CurrentVersion` and `CurrentVersionCode` unchanged. Those specify which version is the stable/recommended build.
4. Commit the changes. Message: "Update K-9 Mail to $newVersionName"
5. Run `fdroid build --latest com.fsck.k9` to build the project using F-Droid's toolchain.
6. Push the changes to your fork of the *fdroiddata* repository.
7. Open a merge request on Gitlab. (The message from the server after the push in the previous step should contain a URL)
8. Select the *App update* template and fill it out.
9. Create merge request and the F-Droid team will do the rest.