Hello @Mrinmoy,
welcome to our o2 Community 
As i can see, @blablup was able to give you some helpful tips already. Have you already managed to contact our English support?
If o2 DSL is not available at your new adress according to this availability check you can cancel immediately with the three months term as written above.
We need a written cancellation then, including a copy of your registration certificate from the Einwohnermeldeamt at your new adress.
You can send it to
Telefónica Germany GmbH & Co. OHG, Kundenbetreuung, 90345 Nürnberg
or via Fax: 01805571766
I have another alternative for you: you also have the possibility to transfer your contract to the new tenant in your old appartment, if he or she is interested. The contract transfer can be carried out online and is charged with 34,99 EUR to the new contract owner.
Please let us know if you have any further questions, we are happy to help you!
Best regards,
Giulia