No download required. Connect your iPhone to a Mac running Catalina, Big Sur, Monterey, Ventura, or Sonoma. The flashing interface is identical to iTunes.
: A versatile alternative that supports a wide range of iOS devices, including Apple TV. iphone firmware flash tool link
If official tools fail or you need advanced options (like "Quick Flash" or retaining data), these are the most popular alternatives: 3uTools (Windows only) No download required