Sorry you are having trouble. Typically the only reason you see the "Can't execute Navigation program" error is if the SD card is not in the iFly, not inserted completely, or the SD has been erased. Make sure the SD card is firmly inserted into your iFly. You really should not ever download the update directly to the SD card, if you accidentally delete files from the SD card, it likely won't work. We would them have to send out a replacement SD card. We make sure that nowhere in the update instructions, to tell download or remove anything from the SD card.
For the iFly 700 and the 720, You can do an update without getting to the main menu by using the below update initiation process. Try downloading the update to the USB Stick. Leave it as a Zip, please do not open it, just "save as" and save it to the USB stick. Then, with the power for the iFly Off (unplugged), insert the USB stick with Update zip file on it into the iFly, then power the iFly on. It should auto find the update and prompt for an update.
If this doesn't work, you may have deleted or removed a file needed from your SD card, very important to not mess with files on the SD card. This is where all data and the navigation program reside.