dpkg-deb -R package.deb packagename
The above commands will create a directory named 'packagename' and extract the contents of the package inside this directory. Do the changes you need to the fake file system inside the 'packagename' directory, and then use dpkg-deb again to build a deb package from the extracted package:
dpkg-deb -b packagename packagename_version_architecture.deb
(Of course, substitute 'packagename' for the real package name).