repeatedly during startup to open the Startup Menu, then select for BIOS Setup or for Network Boot if supported. HP Support Community Troubleshooting & Recovery Z240 - BIOS updates? - HP Support Community - 9094320

If you cannot find a pre-made file, you can build your own "top" file. This is advanced but guarantees safety.

In these scenarios, a standard HP ROM file will not work. You need the (a raw dump) to write directly to the chip via a hardware programmer.

If you share the of the hex dump (e.g., first 256 bytes), I can help interpret: