Postman's default Accept header is set to */* which basically means Postman can process all types of requests, and it is recommended that if you need to specify anything different, remove or untick the old one, and add a new Accept header.
And to add a header to a request in Postman, you need to add it under the Headers tab, from there you can add a new Accept header and untick the default one. The default Accept header cannot be altered so you'll need to add a new one and untick the default.