if you have a lawyer, ask him/her to send a letter to the court taking back your appeal. The lawyers mostly have an experience with the responsible court, if they have many appeals in work there. If you have no lawyer, just send a short letter with the number of your court case - so called Aktenzeichen, or Geschäftszeichen (you have it if you've already got some letters from the court after your appeal) telling them, that you take your appeal back. It must be written in German, your social worker or somebody else can help you to do this. Ask the court for a written confirmation, you can also try to ask for a quick decision, especially if you have some reasons (for example a family member is very sick in your country etc.). If you don't have the number of your case, send them this letter together with the copy of your appeal.
Bring the copy of this letter to the center for voluntary return to inform them, they will see, that your decision is serious. How long it takes your court to stop your procedure and to make a corresponding decision, nobody can tell you in general, each court works with its own speed. I don't think that it will take long. Good luck!