Here's an explanation of how to burn (or flash/program) a new bootloader onto your Arduino UNO, Mega or Nano board:

1. First, ensure you have the necessary equipment and materials:

- A USB cable

- An Uno, Mega, or other compatible AVR-based Arduino board that already has the bootloader installed on it (this will act as the programmer)

- The board for which you want to install a new bootloader

2. Next, download and extract the appropriate Arduino IDE from 'www.arduino.cc'(http://www.arduino.cc) if not yet done.

3. Connect your UNO, Mega or other compatible AVR-based Arduino board with the already installed bootloader (the programmer) to your computer via USB cable.

- Make sure that you have selected the correct serial port in the IDE preferences under 'Hardware' > 'Arduino Uno etc.'

4. Then, connect the target board for which you want to install a new bootloader to the programmer using an ICSP programming cable.

5. Open up Arduino IDE and go to 'Tools' > 'Board:...' Select your board (Uno, Mega, Nano etc.) from the list.

- Make sure that the correct serial port is selected in the preferences under 'Serial Port.'

6. Now, click on 'Burn Bootloader.' This will start flashing a new bootloader onto your target board.

7. Once finished, you should see an LED light up or blink on your programmer's ICSP pins indicating successful programming.

- If not, double-check all connections and try again.

8. Finally, disconnect the boards from each other but keep them connected to power via USB while uploading sketches using serial communication as normal.

Arduino mega r3 pinout