Replace (Change) string variable value in JSON string using C# in ASP.Net

hello

i have this json header

i want to replace "variable here" value with variable data

string Myvariable="This has to replace with static data";

instead of Vriable here it should show variable data MyVariable

 request.AddParameter("application/json", "{\"to\":"+ mobilenumber +",\"from\": \"abc\",\"content\": \"Variable here\"}", ParameterType.RequestBody);