The proxomitron, how do i change incomming headers???

i have the proxomitron and i want to change a incomming header, ive tried on my own but couldnt figure out how to do it change a incomming header.

i have this header:

GET /chat/gfx/actors/white/boy/clothes/945.gif HTTP/1.1
Connection: keep-alive
User-Agent: Mozilla/4.0 (Windows XP 5.1)
Host: 80.69.224.158
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
If-Modified-Since: Thu, 20 Feb 2003 17:30:00 GMT

and how am i gonna change it to:

GET /chat/gfx/actors/white/boy/clothes/47.gif HTTP/1.1
Connection: keep-alive
User-Agent: Mozilla/4.0 (Windows XP 5.1)
Host: 80.69.224.158
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
If-Modified-Since: Thu, 20 Feb 2003 17:30:00 GMT

its only the .gif file im gonna change from 945 to 47 how do i do it???

thanks,
DDingo