Use the sed -i command

sed -i.BAK 's|foo|bar|g' files # -i does in-place replacement
perl -pi.bak -e 's|foo|bar|g' files                # or
perl -pi.bak -e 's|foo|bar|g' `find /pathname -name "filespec"`

Perl is the preferred way, but on some production system you must use the old good sed. For some tips on sed take a look to its faq. Another very powerful command is xargs. Xargs is a rapid  way to process files containing spaces, using  a  combo with find:

Test driven development is a beast to tame. I like TDD but in the past it was quite difficult to achive, at least in a pure-consultant approach with a "turn key" framework behind.