I will give an example of updating the firmware JXD P1000:
1) Download the firmware and firmware utility.
Included with the utility is a driver, you must install it.
Official firmware download page: http://www.jxd.hk/?s=JXD+P1000
2) Run the utility “SP_Flash_Tool” and select “Scatter-loading” pointing to the file “MT6575_Android_scatter.txt”.
3) Press “Download” and connect the disabled JXD P1000 with a USB cable, the firmware will automatically download to the device, a green circle will appear at the end with the words “Download OK” and then you can disconnect the cable from the device and turn it on.