edit your /etc/apt/sources.list file and change "stable" or "woody" to "testing" or "unstable", example:

Change the line:
deb ftp://ftp.debian.org/debian woody main contrib non-free
to:
deb ftp://ftp.debian.org/debian testing main contrib non-free
or:
deb ftp://ftp.debian.org/debian unstable main contrib non-free

HTH

rock