Answer:
Put your address and the date at the top right.
In the top left put their letter reference number.
Below these place a centred heading which says:-
"Request To Defer My Admission"
Then start:
Dear Sir/Madam,
In reference to your letter of (enter the date of your letter of admission offer) offering me admission to your school/academy/company etc, I would respectfully like to request that you allow me to defer my admission by (say how long) for the following reasons:-
List your reasons and explanation.
Thank you for your attention, I look forward to hearing from you shortly with your answer.
Yours faithfully
Your name.