[O] Changing packages and their order in LaTeX export

[Dror Atariah]

I need to include 'amsmath' package in my LaTeX export. Using #+LATEX_HEADER: 
\usepackage{xyz} adds it at the end of the list, and then there are LaTeX 
related problems.

I tried to find where the default list is defined - it seems like it is in 
org.el. However, I have several copies of this file, and I don't know which one 
is the right one to change.

So, two questions:
1. Should I change the org.el or is it somewhere else?
2. If it is the org.el, then how can I tell which file is the one which is 
used? Should I restart emacs after each edit of this file for the changes to 
take effect?

Thanks!
Dror Atariah