#####replace string in a file #####
sed -i 's/original/new/g' file.txt

Explanation:
    sed = Stream EDitor
    -i = in-place (i.e. save back to the original file)
    The command string:
        s = the substitute command
        original = a regular expression describing the word to replace (or just the word itself)
        new = the text to replace it with
        g = global (i.e. replace all and not just the first occurrence)
     file.txt = the file name
#####replace string in a file #####