<p>How to convey a bad news to the client?</p>

What type of bad news?
People in general like to break bad news by stating that the corrections / new timelines will be in place much earlier than we truely can accomplish. Be honest up front and dont give false hope. One instance of bad news is enough... no need to create new promises that you cant keep in the future.
Bottom Line: be honest to your client and yourself to control expectation management.