Format your USB stick Fat 32 then make /et9000 folder in the root and copy the following 4 files to that location.
1) boot.bin
2) kernel.bin
3) rootfs.bin
4) imageversion
Insert USB stick into either USB socket on the rear or front and then Power Off and Power On.
After power-on, ET9000 is detecting USB driver and perceiving the upgrade files existed.
Taking a look at ET9000 informs "image found?" on the left of front panel and pressing "OK" button on the right of it.
ET9000 gets into a state of upgrading the files and displaying "flashing" on it. after complete upgrading, rebooting it.
-Note-
During upgrading, please don't unplug power or shut down STB.
EDIT: RECEIVERS USING THE LATEST BOOTLOADER ALREADY FLASHED WILL USE A FOLDER NAMED et9x00