14.19Example

First:

{
  "customerId": "IDEMP-100",
  "companyName": "Acme",
  "phone": "111"
}

Query Salesforce:

Id = 001ABC
Phone = 111

Second:

{
  "customerId": "IDEMP-100",
  "companyName": "Acme",
  "phone": "222"
}

Verify:

Id = 001ABC
Phone = 222
count = 1

That proves both update behavior and duplicate prevention.