Using backports is simple.
Add this line to the /etc/apt/sources.list.
deb http://www.backports.org/debian etch-backports main contrib non-free
apt-get update
NOTE: An error message might be seen:
Reading package lists... Done W: GPG error: http://www.backports.org etch-backports Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EA8E8B2116BA136C W: You may want to run apt-get update to correct these problems
sudo apt-get install debian-backports-keyring
To install a newer version of nmap:
sudo apt-get -t etch-backports install nmap
NOTE:
WARNING: It you forget to tell apt-get that nmap is installed from backports, and run another apt-get install nmap, it will remove the backports package and install the standard one.