More precision in Altitude Hold

I am new in firmware editing of ArduPilot.
I am using PX4.
Instead of using ardupilot I am using ardusub for my Underwater ROV. Unfortunately, the altitude hold in the firmware fluctuates around 1 meter. The files are almost the same structure as the Ardupilot.

I am trying to find out, which files should I edit on ArduPilot (In my case Ardusub- which is almost the same) to play with the range of Altitide hold mode.
If someone can show me any instruction even on ArduPilot, I hope I can figure it out on ArduSub too,