Wait, you just want to update MC20? I assume you already have it installed? If so the most easy way is to use the following commands in a terminal window...
1) First, we need to cd into the downloads folder by typing the following into a terminal;
cd ~/Downloads
2) Next, we're going to download the latest build. Adjust the link/build number for newer builds as they're released;
wget http://files.jriver.com/mediacenter/channels/v20/latest/MediaCenter-20.0.31-1.deb
3) Finally, this command will install the latest build, again remember to adjust the build number to the latest build as they're released.
sudo dpkg -i MediaCenter-20.0.31-1.deb
If all goes well, you should have the latest build of MC20 installed. If you experience any dependency errors, just use the following command to take care of them;
sudo apt-get install -f