What happens if I don't respond to a demand letter by an attorney on time, or a day late? EDIT: They gave me a four day deadline

This is to all those law people. My ex-employer wants me to keep quiet of her unethical behaviour at the job. I quit and they falsely accused me of stealing. I got a demand letter basically to promise I did not take anything and to not ruin the company's reputation. However, they gave me a four-day deadline. This is U.S. law btw.