We are preparing a web where we explain this! In the mid time, download F-Droid app and go to Settings -> Repositories and add "https://apt.izzysoft.de/fdroid/repo" as new repository. This way your app will be up to date.
Alternatively you can manually download the latest version from the repo https://github.com/KoalaSat/nostros/releases/tag/v0.3.0.4-alpha