Plasma 5.22 is available in PPA Beta for the Kubuntu 21.04 and 21.10 development series.
Warning - This is a beta software and is being released for testing purposes. If you are installing Plasma 5.22 Beta, you should be prepared to encounter errors that may interfere with your daily computer use.
Add PPA and then upgrade:
sudo add-apt-repository ppa: kubuntu-ppa / beta && sudo apt full-upgrade -y
In case of problems, use ppa-purge to remove PPA and to return / downgrade packages.