Upgrade Firmware from rak2245

Is the only way to Upgrade the Firmware from rak2245 by loading the whole IMG to the sdcard including the Pi os?
Or is there another way?

You could install the needed software from another source.

What do you hope to accomplish by “upgrading”? I’m not aware of the actual functional code, which comes from Semtech and not RAK, having had any recent seriously necessary changes made to it.

It seems quite unlikely that the gateway software is at issue in the other thread you have going, most likely this thread should not have been created.