The legal situation
If you sign a contract on the doorstep, the contract is valid, but you can usually cancel it. If you cancel the contract, you are no longer bound by it. In principle, you have 14 days from the conclusion of the contract to do this. However, if you have purchased a product, the period does not start until the product has been delivered.
Sometimes what appears to be one contract is actually two or more contracts hidden in one document. For example, if you buy a product but cannot afford it with your own money, you will be offered a loan. This loan to pay for the product is an separate contract. If you want to cancel it, you have to cancel both contracts.
If the consumer has not been informed about the right of withdrawal (i.e. if you have not received information about the withdrawal procedure by mail or letter), the right of withdrawal expires after 12 months and 14 days at the latest. The withdrawal must be notified to the other party - so it is best to write a letter stating that you are withdrawing from the contract. It is safest to send this letter by registered mail ("Einwurf-Einschreiben"), not by e-mail. This is because you will need proof that the other party has received your notice. It is not enough to simply return the goods. You do not need to give a reason for the cancellation.