Once in a while I need to update my asciidoctor and asciidoctor-pdf installation. Somehow I seem to always forget how to do so. This is a small reminder which will hopefully save some time next time I need to repeat the actions.
One way would be to re-install asciidoctor by a

sudo gem install asciidoctor

This will install a new asciidoctor version, but then I’ll need to clean up the old installation by

sudo gem cleanup asciidoctor

And I would also need to update other packages such as asciidoctor-pdf manually.

A much better way is to update the asciidoctor installation, which also updates other gems as well (such as asciidoctor-pdf). To do so simply

sudo gem update asciidoctor

UPDATE on March 1st, 2017: sudo gem install fails on El Capitan, in this case try…

sudo gem install -n /usr/local/bin asciidoctor