Downgrading BIOS on HP Spectre 15?

[y2kk]

Hello guys, I want to downgrade the BIOS on my HP Spectre x360 15-df1xxx . Of course HP doesn't allow downgrades, so I have probably to use a hardware flasher. Didn't find any possible way to do it through software. I never did this before and I'm not sure if it is even possible at all. I want to downgreade to F.20 Rev.A or even earlier. I'm not sure yet which version is the best, since I have very bad thermal problems with F.26. Also I would like to undervolt so maybe I have to get an even older BIOS version.

BIOS Vendor: AMI
BIOS Version: F.26
Mainboard: HP 863E
Flash module: ABSOK SOIC8 SOP8 Flash Chip IC CH341A (SB Programmer+SOP8 Clip+Adapter)

https://ftp.hp.com/pub/softpaq/sp100501-...100748.exe

If I extract sp131758.exe I get four files: 0863E.bin, 0863E.sig, 0863F.bin, 0863F.sig. I guess I only need the first two files. How do I go from here?  Can you recommend me any guide?