Firmware update failiure

Hi

Have tried updating the firmware on my NVR [DS-7604NI-E1 / 4P] which is currently running [V3.2.1, build 150304]
downloaded the latest firmware off the Hikvision Eu portal [V3.4.91build161220] and tried updating using a usb.
I keep getting the error 'upgrading failed, firmware mismatches'
Also tried using an older firmware which gives me the same error, the NVR was purchased as a UK spec

Any help would be appreciated
Thanks