Upgrade macOS Sierra 10.12.6 to 10.13
I currently have macOS Sierra 10.12.6 and have been trying to update to 10.13 however I cannot seem to find where I can download it. I tried App Support https://support.apple.com/en_AU/downloads/high%2520sierra and they have all versions other than 10.13, and all other versions require 10.13. I am obviously looking for the wrong thing! I have a MacBookAir mid 2013 so according to the requirements I should be able to download it if I knew where it was.
MacBook Air