From Bitpost wiki
Install another OS on an existing VM
DOH you dumbasses...
- Put the ISO on a location that vSphere can see
- Stop the VM
- VM > Edit settings... > Virtual Hardware tab > Configure the CD to mount the ISO, check the box to Mount on boot
- VM > Edit settings... > VM Options tab > Boot options > Boot delay > 3000ms or more so you can get to the boot screen
- Hit F2 to go to "BIOS", change the boot order there to push the CD to the top