Box not flashing?
When you see USB UPDATE on the front of the box you must press the power button on the front of the box for the flash to start.
If your box is not flashing and just going straight to Boot first check the USB memory stick is formatted to FAT32. If you don't know how to do this use the video on this page for either PC or Mac.
If your memory stick is in formatted to FAT32 and still wont flash then the box does not like the memory stick. This is a very common problem and is hard to say which ones it will like, or not like. Either try another memory stick, or buy a branded 4-16GB stick from a reputable retailer, or try the alternative flashing method.