Based on http://blog.fili.nl/updating-a-debian-package-with-a-new-upstream-release/

wget {URL OF NEW UPSTREAM SOURCE TAR.GZ}

sudo apt-get install devscripts
sudo apt-get build-dep {PACKAGENAME}
sudo apt-get source {PACKAGENAME}

cd {PACKAGESOURCE-DIR}
uupdate ../{NEW UPSTREAM SOURCE TAR.GZ} -v {NEW UPSTREAM VERSION}

cd ../{NEW UPSTREAM PACKAGESOURCE-DIR}
dpkg-buildpackage -us -uc -nc

#This may warn about issues with genchanges, but if the .deb file is created, you do not need to care about it.