You can change it manually:
private void UpdateConfigFile(string appConfigPath, string key, string value)
{
var appConfigContent = File.ReadAllText(appConfigPath);
var searchedString = $"<add key=\"{key}\" value=\"";
var index = appConfigContent.IndexOf(searchedString) + searchedString.Length;
var currentValue = appConfigContent.Substring(index, appConfigContent.IndexOf("\"", index) - index);
var newContent = appConfigContent.Replace($"{searchedString}{currentValue}\"", $"{searchedString}{newValue}\"");
File.WriteAllText(appConfigPath, newContent);
}