RPL is a very useful utility to have around:

http://www.laffeycomputer.com/rpl.html

For FNAME in FILE_NAMES

do

sed -i 's/RTG78/NEW_STRING/g $FNAME > /tmp/o

mv /tmp/o $FNAME

done

With GNU utilities:

find . -type f -exec grep -lZ RTG78 {} + | xargs -r0 sed -i 's/RTG78/something/g'

If there may be binary files, you may want to use perl instead of sed:

find . -type f -exec grep -alZ RTG78 {} + | xargs -r0 perl -pi -e 's/RTG78/something/g'

(or use perl in anycase)

You can use SED command

sed 's/search-string/string-to-change/g' file-name >new file