Before we get started:
It is important to ensure you have valid backups of your virtual machines before you make any changes to a Production VMware vSphere Hypervisor ESXi 5.1 Host server, which is hosting virtual machines. Additionally, all virtual machines must be powered off and the server should be in maintenance mode.
1. Download the ESXi 6.0 Offline Bundle from VMware
2. Upload the offline bundle to an ESXi 5.1 datastore
3. Connect to the ESXi 5.1 host server via SSH
At the console prompt, type vmware -l to confirm the ESXi version. VMware ESXi 5.1.0 GA or later should be returned, though this will depend upon whether any patches that have been applied.
# vmware -l
4. Use esxcli at the console command line to upgrade the server
esxcli software vib update -d /vmfs/volumes/datastore1/VMware-ESXi-6.0.0-2494585-depot.zip
The above command does not produce any feedback if entered correctly. After approximately 20 seconds, the upgrade should be complete. However, we have found if you are using slow USB flash drives and SD cards, this can take considerably longer for the files to be written.
If all is well, you should see an update screen, shown above, though you may have to scroll the screen to see all the information. Check the update result at the top.If the update completed successfully, the system needs to be reboot for the changes to be effective.