How do I install/update to Postgres 9.4?

You can add it from the instructions in the page

http://www.postgresql.org/download/linux/ubuntu/

• Create the file /etc/apt/sources.list.d/pgdg.list, and add a line for the repository

  deb http://apt.postgresql.org/pub/repos/apt/ trusty-pgdg main
  

• Import the repository signing key, and update the package lists

  wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
  sudo apt-get update && sudo apt-get install postgresql-9.4