sed is stream editor, but can edit files directly too, with the following:
sed -i -e 's/foo/bar/g' filename
s is used to replace the found expression "foo" with "bar"
g stands for "global", meaning to do this for the whole line. If you leave off the
g and "foo" appears twice on the same line, only the first "foo" is changed to "bar".
-i option is used to edit in place on filename.
-e option indicates a command to run.