How to upgrade from ESXi 5.1 to 5.5

Shut down VMs running on host.

Enter maintenance mode:

vim-cmd /hostsvc/maintenance_mode_enter

Set firewall rules to make patch retrieval possible:

esxcli network firewall ruleset set -e true -r httpClient

Retrieve a list of available upgrades and look for the most recent one:

esxcli software sources profile list -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml | grep ESXi-5.5

Run the upgrade:

esxcli software profile update -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml -p ESXi-5.5.0-1331820-standard

Reboot.

Exit maintenance mode.

vim-cmd /hostsvc/maintenance_mode_exit