It’s not that easy.
In theory, you should identify the BOOT pin on the H753 uC and hook it via an 1k resistor to 3,3V to hold it high, so the uC boots into DFU mode. Then there’s an utility from ST that will allow you to flash the bootloader.
I’ve replaced the F4 on one of my Pixhawk 1s and had to go through the procedure, but it happened only once, a couple of years ago, and the memory is fuzzy, sorry