Hi:
The HP 6200 Pro business desktop PC series does not have the secure boot option.
Only PC's that came with W8 or newer from the factory would have secure boot, such as the later models of the HP 6300 Pro business desktop series.
To boot from another device (DVD/Flash drive), turn on the PC, tap the ESC key at the beginning of the HP welcome screen to get the menu of options.
Select the F9 boot options menu.
There should be a Legacy boot devices section.
Select the USB flash drive or DVD from that section.
Now this is important...when booting from a DVD you actually have to go to the line below where the DVD is listed (I think it will say SATA 2), select that, and press the Enter key.
The PC should then boot from the DVD.