Often there is already a return label in the package, but this is not mandatory. If this is not the case, you will find the necessary information either in your customer account with the online shop or in their cancellation policy or terms and conditions. Here you will find the specific return address in addition to the cancellation period and also the regulation on who has to bear the return costs.
Very important: The mere return to the online shop does not constitute a withdrawal. The withdrawal has to be made explicitly, either enclose it with the package or communicate it separately - please read this article.
Please make sure that you send your return as a package or registered mail with tracking number and keep the receipt until your issue is resolved. This way, you will make sure to have a proof of return in case the return does not arrive at the online shop. Such proof is also a prerequisite for applying for a refund under your Trusted Shops Buyer Protection.
You can find a list of certified online shops with their contact data here: