Go intoi tthe BIOS setup by pressing the magic key for that early in the boot process. That's del or esc or or f2 or f10 or another key, and maybe the screen even has a message like "Press ... to enter setup." The user manual should say. Check the HP site if you don't have that manual.
Then boot from that disk and follow the prompts to install Windows 8.1. But why not Windows 10?
I need your help. I just acquired a 12.5" HP EliteBook 2570p, Windows 7, Intel Core i7-3520 3rd Gen., 2.9GHz, 180GB SSD, 4GB RAM. I want to replace Win 7 with Win 8.1. I burned a bootable Win 8.1 DVD with ISO from Microsoft but I need instructions on the correct steps to boot the laptop so that I wind up with a Win 8.1 laptop. No one on the Internet seems to offer such help, not even HP! I'm not a techie and I need simple, 1-2-3 instructions.