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Press and hold the record button and power on the phone.
When it starts let go of the record button then you should see a message saying to press any key to enter bootloader, press the action button (center button on the directional pad) and you will then be in the bootloader menu.
You need to be fairlyquick to enter the bootloader and having a storage card in the phone can help to get in to the bootloader as it slows the system down making pressing the action buton easier.