Here is how you can change the message for the dunning case (step by step):
Configuration > Dunning > Click on your dunning process (names may vary)
Then select the case you want to change. I believe in your case it is "ZAHLUNGSERINNERUNG"
By pressing Edit you can change the text.
You can also create a new template and add it.
If you have any further questions or want to get into more detail please send us an e-mail to firstname.lastname@example.org. Thanks!