With that much potentially at stake, you might want to discuss this with a lawyer. many lawyers will do an initial consultation for free or a low charge.
However, you could simply write a letter saying that you do not agree that you owe the money and that you dispute the charge. You may give any reasons why you think you are not liable. It might be a good idea to add that there may be other reasons as well, so you do not foreclose any possible legal arguments that you may learn of later.
Send the letter by certified mail, return receipt, and keep a copy with a note of the date that you sent it. It is not a bad idea to get the certified mail form first and include the certified item number as part of the inside address in the letter. Keep the copy, and keep the receipt with it when you get it.
It is not a bad idea to send a copy by email, noting that it is a copy of a certified letter. This will put a record of the date and time it was sent in the email service provider's records.
Normally it is up to the person who claims another owes money to file suit. If your former landlord takes no action, you need do nothing. However it is a good idea to check your credit reports and see if this was reported as a bad debt. if it is, you can file a statement of dispute with the credit bureaus.
If there is any further correspondence on the matter, be sure it is in writing, and that you keep a copy. If you are called on the telephone about it, send a prompt followup letter summarizing the conversation, and particularly any statements made by the other party, and any agreements reached. Keep a copy, and send a copy by email as well as by postal mail. It should probably start something like "In our telephone conversation on {date} about {topic} you stated ..."
If you are sued you will need to consider whether to retain a lawyer to represent you.