There are 2 ways to do it, either via depthboot(software only, no custom firmware, lots of manual OS prep, 0 risk) or custom firmware(maybe physical, model dependant, no os prep, small risk). For custom firmware you usually have to either bridge an internal jumper, unplug the battery, or build a custom cable, depending on your model.

While it is allowed it's not supported by google.

I would never recommend buying a Chromebook with the intention of replacing the OS unless you're looking for a project or you're getting it for cheap.