How to end the O2 Basic blue contract

Hi, I am an international student and now after completing my studies want to return back to my country on April . My O2 basic blue contract started from 03.03.2015. Last year around January, I wrote a written mail to O2  Kundenbetreueng for the cancellation but they didn't respond to my written mail. So I went to the O2 shop where I purchased the contract but they told me that you have to inform us before 3 months. This year also I missed the deadline to terminate the contract because I was not sure when I will be leaving Germany. So what would be the procedure to terminate my contract. If there is any form/documents I need to sign to terminate and would O2 take any penalties??