just edit your /etc/apt/sources.list file.
Change the words "stable" to "testing"
or "woody" to "sarge". For example :
deb
http://ftp.debian.org/debian/ stable main non-free contrib
deb
http://non-us.debian.org/ stable/non-US main contrib non-free
change to :
deb
http://ftp.debian.org/debian/ testing main non-free contrib
deb
http://non-us.debian.org/ testing/non-US main contrib non-free
or to :
deb
http://ftp.debian.org/debian/ sarge main non-free contrib
deb
http://non-us.debian.org/ sarge/non-US main contrib non-free
The difference is that the first will always point to a testing distro. (sarge can change to stable and another name become the testing)
and the second will always point to sarge.
after that, execute :
"apt-get update"
"apt-get dist-upgrade"
for further information read the apt-howto :
http://www.debian.org/doc/manuals/ap.../index.en.html