Undoing changes

I have a question perhaps a stupid question but how do you undo changes that you made to a config file or how do you get the original back if you didn't back up the original? I have not done this thank goodness but I always wondered about it.

 

This isn't possible unless you did a backup before your changes.

 

There is a chance if the configuration file is from a package that you have installed to your system, doing a reinstallation of this package it should reinstall the configuration file. But as falco said backup and roll