Rest assured that tweaking the Amazon Firestick 4K, Fire TV Stick, or Cube settings with this update method is legal and fast. The hardware is yours to do or download with as you please.
It’s only how you use the streaming Firestick device once jailbroken that can be illegal and may step you into trouble. This is based upon usage and copyright laws in your location. In some jurisdictions, it’s even illegal to stream content with MPAA ratings. Most other regions restrict to adults such as XXX or even some NC-17.