代码如下

public void SetAppSettingValue(string app_name, string value)
{
Configuration config = WebConfigurationManager.OpenWebConfiguration("~");
AppSettingsSection app = config.AppSettings;
app.Settings[app_name].Value = value;
config.Save(ConfigurationSaveMode.Modified);
}

如果不能修改，请设置好相应的权限已供访问，比如everyone

动态更改web.config中的appsetting