If i compose an email and have click send and want to open it back to add cc .How can i do to add cc .?

If you have already clicked Send, the original email has already gone. However, it may have saved a copy in the Sent folder. If so, you can open it, and Fwd it to the next recipient that you want to send it to. (If you do that, it is a good idea to remove the "Fwd" notation at the beginning of the subject line.)