If you wanted to pursue higher education then why did you apply for the job? If applied for the job then why did you appear for interview? If appeared for the interview, then why did you accept the offer letter?
Enhancing one's education is a planned decision. Why this wisdom of taking higher level education dawned on you after acceptance of the offer letter? How can you blow hot and cold? By not joining, you would be letting down that company. Imagine if company had done the same thing to you i.e. issuing offer letter and then withdrawing it.
If you are firm in pursuing your education then send them mail on your inability to take up the job. Before sending the mail, you may give a telephone call and inform them.
Dinesh V Divekar
19th May 2013 From India, Bangalore