Find jellyfin related file in /etc/apt/sources.list.d, edit it as root and try replacing „circle” with „bookworm”.
After that apt update
and retry. If it doesn’t work you can also try replacing it with „noble” but the you might also need to replace debian -> ubuntu, but that’s just my guess
PopOS is Ubuntu LTS so it’s only getting ancient KDE Plasma and no, not worth it.
Aside, yes it’s fairly easy, but you might need to change couple of settings regarding theming and then roll back to defaults to un-fuck some desktop theming.