Use sed to replace old strings with new strings in a file:

sed ‘s/old_string/new_string/’ file > newfile
mv newfile file 

or, if you are willing to live slightly dangerously, replace the file directly:

sed -i “” ‘s/old_string/new_string/’ file

If you want to replace all occurences of old_string, you need to use “g”

sed -i “” ‘g/old_string/new_string/’ file