https://www.kpn.com/zakelijk/opzeggen.htm

or 

https://www.kpn.com/service/administratie/opzeggen.htm

Hi @EAN, welcome to our forum.

I understand that you want to cancel your subscription when you're moving abroad. Did you succeed with the links to our website, shared above by @EAZ1964? 

If not: The department in charge of terminating the contracts is not active on this forum, however you could call them using the free telephonenumber 0800-0402 within the Netherlands, and otherwise +31 6 1200 1200. On business days they’re available between 8:00 - 20:00 and on Saturdays between 9:00 - 17:00 (Dutch timezone).