How to initiate USB UPGRADE depends on how you flash (USB-connection or USB-stick) and whether the box is set to standby or to shutdown.
If you shutdown your box, your power button light will be on (blue colour). In this case flashing via USB-connection can be started by just pressing and holding the power button until USB UPGRADE appears on display.
In standby mode (or when flashing from USB-stick) you will have to press and hold power button, in addition press Up key and release power button, when USB UPGRADE appears.
If you're flashing via usb-connection, first flashtools 1.02 and usb-driver for vesta must be installed on your windows pc - otherwise the box won't be recognized.