In addition to the existing stable repository Flathub offers a repository for beta builds. This isn’t meant to be used for nightly builds, but for releases that has some level of testing and are expected to mostly work and be usable to non-developer end-users.
Adding flathub-beta:
flatpak remote-add --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
Note: --if-not-exists is not really needed
Install an application:
flatpak install flathub-beta org.godotengine.Godot
Note: --user installs the application for only the current user, others cannot see/use the app!
Alternatively, you can use a flatpakref, which are generated for each app:
flatpak install https://flathub.org/beta-repo/appstream/org.godotengine.Godot.flatpakref
If you install both the beta and the stable version of an app then they will be installed in parallel. However, only one will be showed in the menus. You can switch which one is currently showed like this:
flatpak make-current org.godotengine.Godot *[beta|stable]*
But from the command line you can always start any installed version explicitly, like this:
flatpak run --branch=beta org.godotengine.Godot
or
flatpak run org.godotengine.Godot//beta
Removing flathub-beta:
flatpak remote-delete flathub-beta