Carol,
 
Actually, playing a conch works more like playing a bugle (no valves)
and yes you change the pitch by changing the aperture formed by your
lips -- like a typical brass instrument.  It will often help if you make
sure the hole in the spire is of suitable size for you to play it
easily.  I personally would try for at least 15mm.  It will also help to
clear out a little of the inner spire to leave more of an open chamber
for the sound to originate.
 
Tom Eichhorst in New Mexico, USA (an old trumpet player who has messed
with playing a couple of conch, helmet, & whelk shells)
 
 
Carol Simpson wrote:
>
> Is the pitch changed when you blow a conch just like it is with a trumpet?
> I can only get one sound so far from mine.