Olson DB: which version in PostgreSQL?
Currently (Jan 23, 2017) 


PostgreSQL uses IANA (Olson) version 2016j
2016j is the latest version.
tz-updates are rolled out to supported major versions in point releases. 9.2 is still supported (current as of Jan 2017) with TZ upgrades.


You can see the change long for the tz database upstream here


  
  Where can I find any information about the timezone's DB version used?
  


check out the README in src/timezone/README which says "the code in this directory is currently synced with tzcode release 2016j."


  
  Is it possible updating the timezone's DB by replacing the newest official IANA version? Consider a legacy system with Postgres 8.4 and no chance of updating it.
  


It's already the latest version, but I imagine if it ever falls behind patches would be accepted. You can find the instructions to create the patch in the README file above.