Assuming that a file file.txt you want to replace where
new, you can do
$ sed 's/old/new/g' file.txt
this way, known as preview, just prints the result of the replacement but not overwrite the file. To overwrite, use the param -i
$ sed -i 's/old/new/g' file.txt
or, if you want to do a backup from the file before replacing, you can use the param -i.bkp
$ sed -i.bkp 's/old/new/g' file.txt
this way it creates a file file.txt.bkp with the old version of the file.
In some more complicated cases you can use regular expression to replace data.
Get this in MAC OS?
command c expects \ followed by text
$ sed -i '' 's/old/new/g' file.txt
This happens because sed in MAC OS is a different BSD flavor.