How do recompile source files of a precompiled headers?

Hi,
I am making a mod using a program called Source SDK with Visual Studio 2008 Professional Edition on Windows XP. With the code package comes precompiled headers. Some of those precompiled headers' source code are enclosed with the project, for example one called tier1. I want to modify a code of a function that belongs to tier1. However, that part of the function is inaccessible via the mod's original project. When I do a Crtl+Shift+F(function name-to find the definition), nothing comes up. However, when I use windows explorer to find the file, I can find it.
I modified the file through Visual Studio, but the changes do not take effect.
I was told the function was part of a precompiled header.

How do I include that file to be compiled with the project?


Here is my original post: I thought I may get extra help here, cuz I really need it.
http://forums.steampowered.com/forum....php?t=1313420