As @Ollie says you'll need to be in bootloader mode - you can't just connect to a device running normally and enter bootloader mode. See here for a bit more info: http://www.espruino.com/MDBT42Q#firmware-updates
You can enter bootloader mode with some other devices but this was actually a conscious decision. Since Espruino is open source, the 'private key' for firmware uploads has to be public. This means that if you could put the device into bootloader mode without any manual intervention then you'd be able to upload whatever code you wanted to any Espruino device - which would obviously be a huge security issue.
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