Responses to offers of admission must be made online as indicated in the instructions contained in your offer letter (posted within your online application).
An applicant is free to accept and decline offers in writing on or before April 15.
An acceptance given or left in force after April 15 commits the student to that offer. The student may not accept another offer without first obtaining a written release from the institution to which a prior commitment has been made.
These requirements are consistent with the Council of Graduate Schools Resolution Regarding Graduate Students, Fellows, Trainees and Assistants.
If a response to an offer of admission is not received on or before April 15, that offer may be rescinded with or without notice to the applicant.