Change UEFI to Lagacy BIOS
Personally I can not find any reason to change UEFI to Legacy BIOS in HP servers for VI,but just in case for any reason you needs to change UEFI to Legacy BIOS (just like PXE boot for some MS product) you can follow below steps.
1.Power on the server,press F9 to get BIOS,select system configuration
2.Select BIOS/Platform Configuration
3.Select System Options
4.Select Network Boot Options
5.Verify only one NIC is enabled then hit esc to back out to BIOS
6.Select Embedded UEFI Shell
7.Change Embedded UEFI Shell to Disabled
8.Select Boot Options
9.Select Boot Mode, hit enter to get rid of warning
10.Change Boot Mode to Legacy BIOS Mode and then reboot server
Just keep in mind to change BIOS before installing any OS or Hypervisor.