Good morning, faslb.

The best way to install Manjaro Plasma "KDE" is by downloading and formatting your device. You can download the ISO from the official website: https://manjaro.org/download/
Choose the "PLASMA DESKTOP" version. You'll need a minimum 8GB USB drive. You can use the Ventoy tool to install Manjaro. You can find more information about it here. https://slimbook.com/en/blog/guides-2/post/how-to-create-a-windows-bootable-usb-from-linux-with-ventoy-384

There's also another alternative: installing Plasma directly on your current Manjaro installation. Take a look at their website: https://wiki.manjaro.org/index.php/Install_Desktop_Environments

Best regards.

Thanks.

I reinstalled the whole system, which worked nicely.