C# Update API 3 Error "Unsupported Media Type"

[S]

Hi - Apologies if the question is duplicate

I want to update jira issue using API 3 

My URL

https://Holy.atlassian.net/rest/api/3/issue/53271

My Json

"update": {
"summary": [
{
"set": "textdfdf dfd"
}
]
}

I have also tried this

{"update":{"fields":{"summary":"jhbsd djdf"}}}

But none works... I keep getting error message 415 Unsupported Medai type

There are various customfields I have not included in the aboe example as the basic summary itsself is not working so...