Change the owner of the subscription or email of the developer

@NICOLE LEUNG (BA-ISD-OOCLL/HKG) As of now this is not possible from azure portal to update the Owner for a particular subscription and update the email ID of the existing user.

To update the owner of the subscription you need to leverage the Subscription Update REST API. This API needs the sid and you can get the sid field from Subscription List API. The name filed in subscription list API can be used as sid for Update REST API.
To pass the ownerId in request body you can get it using User - List By Service API.

Request Body to update the owner details using Subscription Update REST API.

{  
   "properties":{  
      "ownerId":"/subscriptions/{subscriptionID}/resourceGroups/{resourcegroup}/providers/Microsoft.ApiManagement/service/{APIM Instance name}/users/{your user}"  
   }  
}  

Similarly, to update the email ID you can leverage the User - Update REST API

Hope this help you. Feel free to get back to me if you need any assistance.