In this previous post, I have explained how to install an update to ESXi when you are offline.
Find below the instructions to use when your ESX host is connected to the internet:
Login to ESX via SSH (e.g. with PuTTY)
Run the command to allow the traffic to the Internet
esxcli network firewall ruleset set -e true -r httpClient
Run the command to install the update
esxcli software profile update -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml -p ESXi-5.5.0-20140902001-standard
Wait until this completes successfully (this can take a while).
Shut down all guests.
Put host in Maintenance Mode.
Reboot VMWare ESXi