Try this:
Press and hold the backup button on your Sky box (not the Sky remote).
Now try to switch your Sky box back on at the mains. You need to keep holding down the backup button and switch it back on at the mains at the same time.
Continue to press and hold the backup button for up to 30 seconds to see if you see all four lights on the front panel of your Sky box illuminate.
Release the backup button. Your Sky box is now in download mode.
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