How to install KDE on Ubuntu

Desktop version of Ubuntu uses GNOME as the default desktop environment.

KDE can be installed and configured to be the default desktop environment on an Ubuntu system. Users will then be logged in to KDE's Plasma by default while still having the option to log in to GNOME when they want to.

1. Launch terminal.
2. Update apt's package list.

   $ sudo apt update
   [sudo] password for user: 
   Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]
   Hit:2 http://jp.archive.ubuntu.com/ubuntu focal InRelease               
   Get:3 http://jp.archive.ubuntu.com/ubuntu focal-updates InRelease [111 kB]
   Get:4 http://jp.archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB]
   Get:5 http://security.ubuntu.com/ubuntu focal-security/main amd64 DEP-11 Metadata [21.2 kB]
   Get:6 http://security.ubuntu.com/ubuntu focal-security/universe amd64 DEP-11 Metadata [35.8 kB]
   Fetched 374 kB in 2s (205 kB/s)            
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   All packages are up to date.

3. Upgrade installed packages using apt (optional).

   $ sudo apt dist-upgrade --assume-yes
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   Calculating upgrade... Done
   0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

4. Install kubuntu-desktop meta package via apt.

   $ sudo apt install --assume-yes kubuntu-desktop
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   The following additional packages will be installed:
     appmenu-gtk-module-common appmenu-gtk3-module apport-kde apt-config-icons-large apt-config-icons-large-hidpi apt-xapian-index ark baloo-kf5 bluedevil breeze breeze-cursor-theme breeze-gtk-theme
     breeze-icon-theme catdoc cdrdao cryfs cryptsetup cryptsetup-bin
   ##### snipped
   0 upgraded, 671 newly installed, 0 to remove and 0 not upgraded.
   Need to get 402 MB of archives.
   After this operation, 1,490 MB of additional disk space will be used.
   ##### snipped

   This will install a complete KDE Plasma/Kubuntu packages and experience on your Ubuntu system.

5. Select Ok using the tab key and enter to configure display manager.
6. Select SDDM or Simple Desktop Display Manager (optional) and click ok.

   SDDM is the default display manager for Kubuntu/KDE.

7. Reboot your system.

   $ sudo reboot

8. Select Plasma when logging in to the system.

   Select Ubuntu or Ubuntu on Wayland (Wayland) if you want to log in to GNOME desktop environment.