sed -i 's/foo/bar/g' FILENAME
-i option makes sure that the changes are saved in the new file – in case you are not sure that sed will work as you expect it you should use it without the option but provide an output filename. The
s is for search, the
foo is the pattern you are searching the file for,
bar is the replacement string and the
g flag makes sure that all hits on each line are replaced, not just the first one.
If you have to replace special characters like a dot or a comma, they have to be entered with a backslash to make clear that you mean the chars, not some control command:
sed -i 's/./,/g' *txt
Sed should be available on every standard installation of any distribution. At least on Fedora it is even required by core system parts like udev.