Replace a word in a string starting with another word

Hi All,

I have a file in which a number of lines are starting with similar first word but different next words.
I want to replace the any nth word(not 1st or 2nd) with another word.

Eg:- My file contains are like this:-
Ram is a boy.
Ram is a good boy.
Ram plays cricket.

Here I want to replace good with bad.

Thanks in Advance,
Mukesh

Give a real example of your input/output. sed 's/good/bad/' works fine with your current one.

 
sed 's/good/bad/g' filename