Install jellytoast from here and it updates through
GNOME Software, KDE Discover or flatpak update like any other app.
Works on x86_64 and ARM (aarch64) — PCs, Steam Deck, and Linux phones.
Install jellytoast (opens your software centre), or:
flatpak install --user https://wolfgangwarehaus.com/flatpak/io.github.wolfgangwarehaus.jellytoast.flatpakref
To just add the source:
flatpak remote-add --user --if-not-exists jellytoast https://wolfgangwarehaus.com/flatpak/jellytoast.flatpakrepo
Everything here is signed; the key is in those two files.