Terminal command to find and replace string in text file

Terminal command to find and replace string in text file :
 

sed -i 's/original_string/new_string/g' text_file_name.txt

Explanation:
sed = Stream EDitor
-i = in-place (i.e. save back to the original file)

The command string:
s = the substitute command
original_string = a regular expression describing the word to replace (or just the word itself)
new_string = the text to replace it with
g = global (i.e. replace all and not just the first occurrence)