To install the Beta (needed once only):

flatpak install --user https://flathub.org/beta-repo/appstream/app.organicmaps.desktop.flatpakref
flatpak make-current --user app.organicmaps.desktop stable

To run the Beta:

flatpak run app.organicmaps.desktop//beta

To update the Beta:

flatpak update app.organicmaps.desktop//beta

References:
https://discourse.flathub.org/t/how-to-use-flathub-beta/2111
https://blogs.gnome.org/alexl/2019/02/19/changes-in-flathub-land/