I'm afraid if you don't know how to find it already then anything I could say to you here is unlikely to enlighten you.
That said, mob.h, line 836:
Code:
int32 cur_hp;
int32 max_hp;
int32 base_hp;
Keep in mind that these values are manipulated all over the place and you would need to change all of those places, and potentially how the data is stored in the database as well. This is not a small task.
You might want to ask yourself first if maybe 2.1 BILLION hit points is enough and if whatever you are trying to achieve could be done a different way to work within that limitation.