Step 1:  Please format your U-Disk to "FAT32" firstly;

Step 2：copy rootfs.sfs and uImage to the U-Disk;

Step 3:  Insert the U-Disk to the USB port of UD851B,and power on or restart,then wait about 45 seconds,Until blue LED on;

Step 4:  pull out the U-Disk and resart UD851B. After the upgrade, please delete the rootfs.sfs and uImage in the U-Disk.