We will start by plugging the KK2.1 board into our computer via the USBasp programmer.
First, plug the 10-pin end of the programming cable into the USBasp programmer board. Second, plug the USB end of the USBasp programmer into an available USB port on your computer.
Now, the third and last step is to plug the 6-pin side of the ASP programming cable into the KK2.1 board. However, the direction of the cable does matter. Fortunately, it is easy to figure out if you have the cable the wrong way, and if you accidentally do plug in the cable the wrong direction, don't worry, no harm will befall your KK2.1 board. So, when you plug the 6-pin side of the ASP programming cable into the KK2.1 board, you should see the KK2.1's screen light up and display the "SAFE" screen. If your KK2.1 does not light up, you have the cable on backwards, so just turn it 180o and everything will be fine.
For Windows users, there is one last thing to do. When you plug in your KK2.1, you will probably notice a message appear on your computer informing you that Windows is attempting to install the driver for your new device. Despite its best efforts, Windows will fail at this task and we will have to give it some help by locating the USBasp driver we downloaded in the previous step:
At long last it is finally time to do the actual firmware updating. So start by running the KKmulticopter Flashtool we downloaded in the previous step (it usually takes ten seconds or so to start). There are a total of five fields in the KKmulticopter Flashtool software we will need to set for the firmware update to work.
Finally, with all the fields set, click the green button on the right side of the Flashing firmware area. The firmware update process can take 20 to 30 seconds or so but eventually you should see a message in the KKmulticopter flashtool indicating that the firmware flashing process was successful.
Congratulations, you have successfully updated your KK2.1 firmware.