@chenga see here.

https://hilfe.o2online.de/english-o2-community-broadband-dsl-70/how-to-do-a-cancellation-of-my-dsl-contract-due-to-moving-abroad-508314

edited: Blablup is right. not read „without obligation“

For Flex contracts (without minimum contract term), the notice period is 4 weeks to the end of a calendar month. You can cancel the contract in text form by letter or fax. If you wish, you can use this form. Please fill it out, sign it and send it to the address given.
No further documents are required for Flex contracts.