Cubeblack bootloader flashing


I need a help / suggetsion the following issue.

The hardware I am using is cubeblack(pixhawk2).
we wanted to put a modified bootloader( modified verion of ardupilot bootloader).

In the cube we do not have any provision of connecting the stlink, please suggest how can we replace the existing bootloader with the new one.
I tried searching for the boot0 pin also but could not find over the board( for using DFU).

Thank you.