The (boot)ROM is on-die, within the CPU, there's no way to replace it. You can potentially replace the OS image (confusingly, often referred to as a "ROM") on

The (boot)ROM is on-die, within the CPU, there's no way to replace it. You can potentially replace the OS image (confusingly, often referred to as a "ROM") on the flash storage, but unless the bootloader agrees to boot it (either due to it being signed correctly, or due to the bootloader being unlocked) you're SOL.