How to reinstall a package in Ubuntu

1. Open terminal
2. Type
   
   
   sudo apt-get install --reinstall {package}

{package} is the package name that you want to reinstall

Example

I want to reinstall firefox

sudo apt-get install --reinstall firefox

Source: http://askubuntu.com/questions/89638/how-to-completely-uninstall-and-reinstall-firefox