Taken from
http://www.ccm.ece.vt.edu/~lscharf/samd/?topic=Linux&title=Search+and+replace+in+multiple+flies :

Search and replace in multiple flies
Keywords:
Date Created: 2002-11-22
Author: Matt Learn and Luke Scharf

Problem: To search and replace a string in a group of files. This is a
classic application of Unix text processing, but what is the best way?

One-line solution: If you have Perl, there is an easy answer:

perl -pi -e 's/<String 1>/<New String>/g' *

This is preferable to using sed because it will write the changes back
to the original file; you don't have to "mv file.fixed file.orig".