I recently updated an ESXi 5.5 host to 6. This is a remote machine so didnt have access to load the ISO. This is how i did it via SSH and ESXCLI.
Running through the ESXi update:
Shut down all running VM’s
Put ESXi host into maintenance mode
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vim-cmd /hostsvc/maintenance_mode_enter
Allow httpClient access out through the firewall.
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esxcli network firewall ruleset set -e true -r httpClient
Run the following command to download and install the update. Make sure you use “update” here DO NOT use “install” as it may overwrite drivers that you need.
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esxcli software profile update -d
https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml
-p ESXi-6.0.0-20150404001-standard
Once that is complete, go ahead and reboot the host
After your host has rebooted, take it out of Maintenance mode
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vim-cmd /hostsvc/maintenance_mode_exit
That should be it.