changing active window

Hello, 

I want to request a specific window becomes active.
After reading the documents, I found _NET_ACTIVE_WINDOW property. 
I changed the property to the window id which I want to see using
XChangeProperty function. But nothing hanppend. 
Can anybody tell me what I'm wrong and how to do? 

-
HeeChul Yun,          
Embedded S/W Team at ETRI
phone: +82-42-860-1673