I need a sim Replacement on a sim that a husband bought and registered for her wife, because the husband is now dead

Hi Kanu, MTN no longer do SIM swap on a deceased subscriber, except if such request is approved by Nigerian Communication Commission (NCC).
Kindly send your request to the Nigerian Communication Commission (NCC); NCC will then send MTN an authorization letter to retrieve/replace the line if the request is approved.
You can contact NCC by dialing 622 Toll Free Contact Center