I think you have to call here:
DB Vertrieb GmbH
Fulfillment Center
Postfach 60 05 03
22205 Hamburg
Telefon:+49 /1805/101111
Maybe you need to fill out this form, too:
REfund PDF Form
And send it to the adress mentioned in the form. Depending on the ticket DB will deduct a refund/cancel fee.