With the release of Media Center 21, the APT Repository has been updated and changed for Media Center 21. What does this mean? If you're already using the APT Repository for Media Center 20 and you're going to replace it with Media Center 21, you need to update your sources. Otherwise if you're doing a clean install of Media Center 21 you can skip step 0!
Step 0 - Replacing Media Center 20 with Media Center 21:
First make a backup of your Media Center 20 library and save it to a place where you can find it, e.g. desktop. This is VERY important!
Now we've got to remove the old mediacenter20.list file from the /etc/apt/sources.list.d/ directory. Open a Terminal (by either pressing CTRL+ALT+T or by searching for the Terminal in your desktop environment) and either type the following command or copy and paste the command in and press Enter (you *may* be prompted for your password);
sudo rm /etc/apt/sources.list.d/mediacenter20.list
Now assuming you made a backup of your library, lets remove Media Center 20 completely. With a Terminal open (by either pressing CTRL+ALT+T or by searching for the Terminal in your desktop environment) and either type the following commands one-by-one or copy and paste the commands in one-by-one and press Enter (you *may* be prompted for your password);
sudo apt-get remove mediacenter20
Then...
sudo rm -rf '/usr/lib/jriver/Media Center 20'
And finally...
sudo rm -rf '~/.jriver/Media Center 20'
This will completely remove Media Center 20 - now you're ready to install Media Center 21!
Step 1 - Adding the JRiver Media Center 21 APT Repository:
The APT repository is required to install Media Center 21 and keep Media Center 21 updated with every new release.
First, open a Terminal (by either pressing CTRL+ALT+T or by searching for the Terminal in your desktop environment) and either type the following command or copy and paste the command in and press Enter (you *may* be prompted for your password);
wget -q "http://dist.jriver.com/mediacenter@jriver.com.gpg.key" -O- | sudo apt-key add -
Next, with the Terminal still open, we need to download and add the correct mediacenter.list file to /etc/apt/sources.list.d/ depending if you're using 32-bit or 64-bit so either type one the following commands or copy and paste the command in and press Enter;
If you're using 64-bit Debian Jessie (or any derivatives like Ubuntu, Linux Mint, etc.):
sudo wget http://dist.jriver.com/latest/mediacenter/mediacenter21jessie.list -O /etc/apt/sources.list.d/mediacenter21.list
Or...
If you're using 32-bit Debian Wheezy (or any derivatives like Ubuntu, Linux Mint, etc.):
sudo wget http://dist.jriver.com/latest/mediacenter/mediacenter21.list -O /etc/apt/sources.list.d/mediacenter21.list
Or...
If you're using an ARM device (like a Raspberry Pi) and distro like Raspbian:
sudo wget http://dist.jriver.com/latest/mediacenter/mediacenter21native.list -O /etc/apt/sources.list.d/mediacenter21.list
Finally, we need to reload the packages list, so with the Terminal still open either type the following command or copy and paste the command in and press Enter;
sudo apt-get update
That's it! Now we can install Media Center 21...
Step 2 - Installing Media Center 21:
You can now install Media Center 21 by opening a Terminal (by either pressing CTRL+ALT+T or by searching for the terminal in your desktop environment) or re-use the Terminal from the previous step. Next either type the following command or copy and paste the command in and press Enter (you *may* be prompted for your password);
sudo apt-get install mediacenter21
And that's it, Media Center 21 should be installed now. Hope this helps people getting the Media Center 21 APT repository added and working on Debian, Ubuntu, Linux Mint, etc.