https://docs.arduino.cc/built-in-examples/arduino-isp/ArduinoISP/
Arduino ISP turns your Arduino into an in-circuit programmer to re-program AtMega chips. What makes an Arduino what it is? Many things, but one of the most important ones is the way every Arduino board is easily programmed with the Arduino Software (IDE). It is enough to connect it to the computer U...
docs.arduino.cc