One thing to be aware of is that letters are not guarantees that they will follow up or take you to court. Especially if they have a weak case - it is just as likely to be a bluff, hoping you'll just stop because they are big and you are small.
Ask an attorney to know for sure, but if they have no case and are just being bullies, don't cave in.
That's dangerous advice, if you have anything to lose, and the C&D isn't obviously complete nonsense. Your adversary's lawyer may have causes of action that he chooses not to disclose just now.
Ask an attorney to know for sure, but if they have no case and are just being bullies, don't cave in.