cd /path/to/your/folder
sed -i 's/foo/bar/g' *
Occurrences of “foo” will be replaced with “bar”.
On BSD systems like macOS, you need to provide a backup extension like -i '.bak' or else “risk corruption or partial content” per the manpage.
cd /path/to/your/folder
sed -i '.bak' 's/foo/bar/g' *