Thanks to https://www.itzgeek.com/how-tos/linux/ubuntu-how-tos/how-to-install-plex-media-server-on-ubuntu-18-04-ubuntu-16-04-linux-mint-19.html

Using Plex Repository

Import the Plex repository’s GPG key using the curl command.

<pre class="wp-block-preformatted">curl https://downloads.plex.tv/plex-keys/PlexSign.key | sudo apt-key add -

Add the Plex repository to your system using the below command

<pre class="wp-block-preformatted">echo "deb https://downloads.plex.tv/repo/deb public main" | sudo tee /etc/apt/sources.list.d/plexmediaserver.list

Now, update the apt index and install the latest version of the Plex Media Server.

<pre class="wp-block-preformatted">sudo apt update
sudo apt install -y plexmediaserver

Plex Media Server package places repository configuration in /etc/apt/sources.list.d directory. Since we already have the plexmediaserver.list in the repo directory, the installer may ask you below questions. Type Y and press enter.

<pre class="wp-block-preformatted">Configuration file '/etc/apt/sources.list.d/plexmediaserver.list'
 ==> File on system created by you or by a script.
 ==> File also in package provided by package maintainer.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** plexmediaserver.list (Y/I/N/O/D/Z) [default=N] ? Y
Installing new version of config file /etc/apt/sources.list.d/plexmediaserver.list ...