https://archlinux.org/news/plasma-640-will-need-manual-intervention-if-you-are-on-x11/
If you are scratching your head on how to do so
- at the login screen → CtrlAltF4
- enter your username → Enter
- enter your password → Enter
run
sudo pacman -Syu plasma-x11-session
then
systemctl soft-reboot