Click on the link in your acceptance email that directs you to an "accept/decline" page. Often this procedure is as simple as checking off a box that says "Decline."
Contact the administrative secretary the department you applied to for your postgraduate degree, either via email, phone or mail. Inform him of your decision. He can tell you if the college requires you to take any further steps.
Speak to the professor who offered you the position. Sometimes departments seek out students for postgraduate programs that they already know from their graduate studies. A professor may suggest a student complete her Ph.D. at that university under his mentorship. A formal registering procedure may follow this informal offer later. Set up an appointment with this professor or contact him by phone or email letting him know your decision.