Since Calibre is updated pretty much every Friday, that day when I start Calibre I look at the right bottom side. There's a link to get the update. From this web page I copy and paste the command in a terminal, enter the sudo password and select /opt as the place to write the software. A couple of minutes later I can run an updated Calibre
Don't forget to close Calibre before the update and you should be good to go.