I installedl KDE-plasma-desktop on top of my already working Mint 21.1 install (tried a fresh kubuntu, but the Nvidia driver wouldn’t work).
Everything works great, but I realized that somehow, from the Mint/Ubuntu repostories, I installed Plasma 5.24.
I read, however, that up to version 5.27+ is available.
I tried adding the Kubuntu backport repository but it made no difference.
How can I install a newer version of Plasma desktop?
The simplest answer is that you install a different distro.
Distro policies are what determine the update schedule; if you’re dissatisfied with your distro’s update schedule, it’s a sign that the distro isn’t right for you. Trying to modify its release schedule by installing 3rd-party repos can work, but it creates a franken-distro that is likely to break in exciting and unusual ways at random times. Then you’ll ask for help, explain your setup, and people will tell you this same thing. 
If you installed Kubuntu but the NVIDIA driver didn’t work properly, that’s the thing that needs troubleshooting and solving. Maybe we can help with that.
Thank you for your answer. I agree with you. I’ll try with Neon, maybe.
I suppose I should start another thread regarding the Nvidia thingy, however, briefly, I installed a few distros on my (11 yo old) Asus laptop over the years and the Nvidia drivers alwasy worked, but in this case I had to give up - Kubuntu 23.04 would hang on boot no matter what (I purged, tried different kernels, different driver versions, using the nvidia .run file as well as the ubuntu reccomended, and more …) It wouldn’t even list my card’s last compatible drivers (418) in the repository.
Try a Arch based distro over ANY UBlunder distro. Here are a few great choices, all using the latest Plasma desktop.
XeroLinux
Garuda
Big Linux
Manjaro
Calam-Arch-Installer