Lets say you're going to do MS/MTech [Post Graduation]
I am really glad to have been offered such a wonderful job at a innovative and unique organization like yours.
I would be proud to be an employee at [Name of the Organization]
However, after a careful and long consideration and advice from friends and family, I have come to the conclusion that I would be better equipped to serve your organization after I have done MS from XXXXX University of XXXXXX.
I really hope to be a part of your organization in future.
Thank you for providing me this opportunity. I regret any inconvenience I may have caused.