Rather than having to go through a failed installation, editing the postinst script then re-installing, here is an alternative method where the package is downloaded, unpacked, the script edited then the package rebuilt and installed in the normal way using apt-get. More here

Having said that, it’s a quick-and-dirty fix-up and as such only really suitable if you only need to make simple changes to the control scripts such as postinst, prerm, etc, in an existing package. Major changes to a package’s structure or contents will need more care and you should take a look at the Debian Maintainers Guide or any of the other HOWTOs and FAQs available on the web for detailed information on how to do this.