Windows: In most cases, you shouldn't fiddle with system files on your operating system. But sometimes you don't have a choice. In those instances, SysMate can help make the process easier.
The app is impossibly simple to use. You point the first box to the file you want to replace and the second box to the new version. No mucking about with command lines or Emergency Repair Disks. It even makes a backup of the original so you can revert the changes if necessary.
Again, this isn't something you should need to do very often, but it's a handy piece of software to have in your tool belt for when you do need it.