string settingValue = ConfigurationManager.AppSettings.Get("setting1");
string settingValue = config.AppSettings.Settings["setting1"].Value;
config.AppSettings.Settings["setting1"].Value = "123";
config.Save(ConfigurationSaveMode.Modified);
App.config
<configuration>
<appSettings>
<add key="setting1" value="123" />
</appSettings>
</configuration>
ConfigurationManager.AppSettings.Set("setting1", "123");