To retrieve a specific Online Meeting instance from a recurring meeting series and update participants, you can follow below steps

• Get the seriesMasterId also known as event_id (the recurring meeting "parent") using GET https://graph.microsoft.com/v1.0/users/{user_id}/events replace {user_id} with UPN

• Get all instances (occurrences) of that recurring event GET https://graph.microsoft.com/v1.0/users/{user_id}/events/{event_id}/instances?startDateTime=2025-06-01T00:00:00Z&endDateTime=2025-06-09T23:59:59Z replace {user_id} with UPN and {event_id} with id from the previous step

• Update the attendees to a specific instance PATCH https://graph.microsoft.com/v1.0/users/{user_id}/events/{instance_id}

  Request body:

{
  "attendees": [
    {
      "emailAddress": {
        "address": "[email protected]",
        "name": "Person"
      },
      "type": "required"
    }
  ]
}

I am able to update the attendees to my intended user as shown in above image.