Upgrade using ISO Disk Image

• Download the VMware-VMvisor-Installer-5.1.0-799733.x86_64.iso from My VMware
• Reboot the host and boot the host from CD/DVD ROM

• Select ESXi5.1.0-799733-Standard Installer

• Select the Storage device on which the installation has to be upgraded

• Select Upgrade ESXi, Preserve VMFS datastore

• Press F11 to proceed with the upgrade

• Enter to reboot after the upgrade has been completed. Please make sure to remove the installation DVD/CD and BIOS has been altered to change the boot order adjust it accordingly

Ta Da!!! you are now running VMWare ESXi 5.1.0 on your host

VMWare 5.1.0 Important Considerations for Upgrading:

http://longwhiteclouds.com/2012/09/12/vsphere-5-1-generally-available-important-upgrade-considerations/