I was going through the discussions to update the bootloader to from 4 to 5. If you are using pixhawk with 2.4.8 which supports 2 mb flash but can’t upload v3 firmware due to bootloader version 4, here is what worked for me: Download any fmuv3 firmware with bootloader which must be less than 1mb if you are using fmuv2. I downloaded ardusub firmware ver 4.1.2 with bootloader. Open mission planner and select custom firmware. select board type fmuv3 and flash this firmware. Reboot and connect by mavlink. Go to install firmware tab and it will now show “update bootloader” click at and bootloader will be upgraded. You can now flash full firmware with all its functionalities.