Hello @xha1e
PoESP32 is a unit and meant to be controlled via Groove serial connection using AT commands.
May I ask what is the reason you'd like to modify the preinstalled AT firmware?
That said, if you look at the schematic you'll see J2 (Download Socket) in the bottom right corner. J2 can be used with an USB serial adapter to modify the preinstalled AT firmware.
Note: J2 is internal and not populated.
Note: the preinstalled internal firmware can be altered / replaced using Arduino IDE. But please be aware that restoring the preinstalled AT firmware requires the ESP-IDF build environment (which is different from the Arduino IDE). Preinstalled AT firmware source code and build instructions can be found here.
Thanks
Felix