No download required. Connect your iPhone to a Mac running Catalina, Big Sur, Monterey, Ventura, or Sonoma. The flashing interface is identical to iTunes.
: Apple only allows the installation of "signed" firmware. Attempting to flash an unsigned (usually older) version will result in an error. iphone firmware flash tool link
Free, Official, Windows & Mac. Official Link: www.apple.com/itunes/ (Windows) or Built-in Finder (macOS). No download required
Always use iCloud or a local computer backup before starting. Find Your Firmware: then Volume Down
If your phone is unresponsive, you may need to put it into Recovery Mode (usually by quickly pressing Volume Up, then Volume Down, then holding the Power button). Select Action: Update: Reinstalls the OS without deleting data.