It depends which kind of contract you have and why you want to cancel.
In general, you may cancel your contract by sending a written cancellation note in German (as German is the sole contractual language) to the appropriate address - either by postal service -or- by using the contact form. This has to be done usually one [in case of an inital runtime of 12 months] or three months [in case of an initial runtime of 24 monts] in advance to the end of your contractual term.
This is the "regular" war and needs no other reasons...
In case of a premature cancellation, you have to send appropriate documents prooving that you are moving to an area where your contract cannot be fullfilled. In that case, your contract will be cancelled by the date you move or the date of your cancellation being received (the later of the two counts) plus 3 months. But this only applies to landline contracts - in contrast, mobile contracts CANNOT be cancelled prematurely!
