Contract termination (english)

Hello,

I have a plan O2 nxt of 24.99€ plus the device plan of 15€. It will end on 07/08/2015. I know I must send some documents to terminate the contract with 3 months of anticipation, in my case in May. But I won't be in Germany from April to July. My questions are:

-  can I cancel the automatic renewal before the 3 months period?

- What documents and letters should I send and where?

Thank you very much.