Since this is a new action and they have not answered, you can modify it without having to ask leave of the court. I am assuming that they are pro se
and there is not an attorney. Here is the requirement for service by certified mail. Note that you have to include a waiver for them to return
(i) Service of Process by Mail. A defendant may accept service of process by mail.
(1) Acceptance of service of a complaint by mail does not thereby waive any objection to the venue or to the jurisdiction of the court over the person of the defendant.
(2) A plaintiff may notify any defendant of the commencement of the action and request that the defendant waive service of a summons. The notice and request shall:
(A) be in writing and be addressed directly to the defendant, if an individual, or to an officer or managing or general agent of the defendant or other agent authorized by appointment or law to receive service of process;
(B) be dispatched by certified mail, return receipt requested;
(C) be accompanied by a copy of the complaint and shall identify the court in which it has been filed;
(D) inform the defendant of the consequences of compliance and of failure to comply with the request;
(E) state the date on which the request is sent;
(F) allow the defendant 20 days from the date on which the request is received to return the waiver, or, if the address of the defendant is outside of the United States, thirty days from the date on which it is received to return the waiver; and
(G) provide the defendant with an extra copy of the notice and request, including the waiver, as well as a prepaid means of compliance in writing.
(3) If a defendant fails to comply with a request for waiver within the time provided herein, the court shall impose the costs subsequently incurred in effecting service on the defendant unless good cause for the failure is shown.