To delete existing content you need to first prepare the request payload and do a POST to the endpoint DeleteContent as part of the body.
{
"slug":"[Your Slug]",
"id":"[The content id]",
"accessToken":"[Your Access Token]"
}
Once the endpoint is called, you will now get a response which will indicate if your content was added successfully.
Please note, the changes will make the Dialogue as a draft version. You will need to deploy your Dialogue for it to be available. To understand further, please have a look at Deploying your Dialogue.