Download and insert the VMware ESXi 6.0 media into the ESXi 5.5 host and boot from the disc drive, you will be presented with the following screen highlight ESXi-6.0.0-2492585-standard Installer and hit Enter.
Hit Enter at the welcome screen.
Hit F11 on the EULA screen to accept the terms.
As ESXi 5.5 was already installed on this host, the setup detected a previous installation. Highlight it and hit Enter.
In this instance, select Upgrade and hit Enter.
Review the confirmation screen and hit F11.
The upgrade process took about 3 minutes installing onto SSD disk.
The upgrade successfully completed for me hit Enter to reboot the host.