LVM - How to Reduce a Logical Volume

[gecarroll at notes.cc.sunysb.edu]

The FC5 install by default creates one logical volume for the entire
system.  If you
wish to reduce the size of this volume to free up space for other logical
volumes,
you must first unmount it, whether you are using the GUI LVM utility or via
the
command line.  But since the volume contains the running system, file are
active
and attempts to unmount fail.  How would you reduce(or increase for that
matter)
the size of a logical volume when it contains all or part of the running
system ?

TIA

George