Copy SD image file to MicroSD Card and insert MicroSD card to tablet.
Step2: Enter boot menu
Power off tablet, then press and hold power key and volume up around 3 sec. to enter boot menu as below
picture. (volume up => move up, volume down=> move down and power key=> select)
Select “apply update from external storage”
Use volume up & down to select SD image you want to flash. Then press power key to run SD upgrade process.