Ahmed2024

So you're getting U03 error code; that means "Document inside the USB disk with wrong format."
As far as I know, the USB stick has to be empty with just one hex file on it, and it must be called "dsp.hex" without the quotes. I don't know how the USB stick should be formatted (NTFS, FAT32, etc). Perhaps others who have succeeded can say.
Be very careful that you have the right firmware update file; I've just realised that using the USB stick method bypasses the "sanity check" for the right kind of firmware (e.g. Kings get a different firmware to others, 2806x processors have their own, etc). In particular, the early Axpert Max E 8kW models had a lot of 90.xx firmwares. Voltronic seem to have stopped using this series, as they were a mix of wildly different models with no pattern. 90.45 might be for a totally different model than 90.46 as a contrived example.