Update 2020-11-03: This site has a lot of great info and links.

Update 2018-08-26: Just run this command from SSH and reboot if you want to upgrade to the latest version. This will also upgrade between major versions, i.e. 6.5 to 6.7.

esxcli software vib update -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml

With Mike’s excellent post, I was able to update my esxi install from Update 1 to Update 2 via the command line.

Summary:

esxcli network firewall ruleset set -e true -r httpClient
esxcli software profile update -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml -p ESXi-5.5.0-20140902001-standard
esxcli network firewall ruleset set -e false -r httpClient
reboot

To get the list of updates:

esxcli software sources profile list -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml