Replacing all occurences of a string in a file with another

In Linux, replacing all instances of a string with another string is easy thanks to sed. A simple example is as follows:

To replace the string foo with the string bar in all .txt files:

This will replace the strings. However, it won't actually save the changes and will instead output the modified text to the screen. To retain the changes or, in other words, to perform the replacements in place within the file, use the -i switch:

Easy peasy Japanese.