There are several methods.
I am not sure where you "are" with your Preenfm2. Is is already running and you
want to upgrade the firmware? Or have you built one and need to flash for the first time?
If your synth is already running, then the easiest method is probably to boot into the bootloader and enable mass storage device mode, so that you can access your usb-stick via the broken out usb port. Then you can simply copy the firmware to the root directory of the stick and update it directly on the Preenfm2.
The docs for this should be pretty clear. You can also put your Preenfm2 into DFU mode and update via terminal with the dfu-util program.