There is a DPCM sample editor available from the 2A03 instrument editor, it's intended mainly for removing silent parts from end of the sample. Other types of edits should be done before the sample is converted to DPCM.

Use the sample view to select a part of the sample and to set the start position (for preview). The buttons explained:

• Delete
  Removes a part of the sample, select the part you wish to remove and press delete. This works best on the beginning and end of a sample; if you need to remove something in the middle then it's usually better to do this on the sample before you convert it to DPCM.
  
  
• Tilt
  Inserts zero-value samples inside a selected area, causing the sample to tilt toward zero. The idea is to restore the delta counter back to zero when the sample is finished, in order to restore volume for the triangle and noise channels.
  
  
• Start from 64
  Draws the sample with an initial value of 64 and is used to center the start of the wave. This is another way to keep the triangle and noise volume consistent, make sure all samples starts and ends at 64. The sample itself is not affected by this option, the equal operation in the tracker is to use the effect Z40 when playing the sample.
  
  
• Play
  Previews the sample at the selected pitch.
  

The offset value tells the start position (as used internally by the DPCM channel), this value can be used with the Yxx command. The pos value is the position in size units as used by the DPCM channel.