How to boot in download mode with a Zonda a450
Okey. As described in the specs, the device has a Spreadtrum chipset (SC7731). There is no boot mode on SPD devices.

To flash a ROM, you need a file in the .pac format and use a flasher (ResearchDownload). 

You MUST install SPD drivers, otherwise the device won't be detected by the computer. 

As there is no download mode, just swicth off and plug the device by holding volume down or up.