Do note that there is a sticky thread in the Related Tools subforum for each OS, offering an automatic updater.
IIRC the OSX one is an app you run manually, I wrote the linux auto-updater (emphasis on
automatic) and ported it to OSX -- see its home here:
https://github.com/eli-schwartz/calibre-installer
And I finally got confirmation that someone uses the OSX version successfully, so I feel comfortable recommending it.