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find ./ -name *.<i>extension</i> -exec sed -i 's/<i>find</i>/<i>replace</i>/g' {} \;
Replace extension of the extension of the files you want to search in (or use any other regex) Replace find with what your looking for Replace replace with what you want to replace your query with.
Warning there is no undo and if your not using revision control or have backups you can seriously mess things up.