First of all, DHL says
The first step is to call on the complaints line 0345 072 0278
Then search these boards for: I used a different courier. You will find many posts with the steps to take to mark your Simple Delivery item as dispatched, change the tracking number, and request a refund (these are linked). Do this within 14-days.
If it was your buyer paid for SD they will be refunded automatically within 30-days.