<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="oracle.manageddataaccess.client"
      type="OracleInternal.Common.ODPMSectionHandler, Oracle.ManagedDataAccess, Version=4.122.18.3, Culture=neutral, PublicKeyToken=89b483f429c47342"/>
    </configSections>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
  </startup>
  <system.data>
    <DbProviderFactories>
      <remove invariant="Oracle.ManagedDataAccess.Client"/>
      <add name="ODP.NET, Managed Driver" invariant="Oracle.ManagedDataAccess.Client" description="Oracle Data Provider for .NET, Managed Driver"
        type="Oracle.ManagedDataAccess.Client.OracleClientFactory, Oracle.ManagedDataAccess, Version=4.122.18.3, Culture=neutral, PublicKeyToken=89b483f429c47342"/>
    </DbProviderFactories>
  </system.data>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <publisherPolicy apply="no"/>
        <assemblyIdentity name="Oracle.ManagedDataAccess" publicKeyToken="89b483f429c47342" culture="neutral"/>
        <bindingRedirect oldVersion="4.122.0.0 - 4.65535.65535.65535" newVersion="4.122.18.3"/>
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
  <oracle.manageddataaccess.client>
    <version number="*">
      <settings>
        <setting name="TNS_ADMIN" value="asd" />
      </settings>
    </version>
  </oracle.manageddataaccess.client>
</configuration>

var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            config.AppSettings.Settings.Add("TNS_ADMIN", @"anylocation");
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("appSettings");

var xmlDoc = new XmlDocument();
xmlDoc.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
var path = @"//oracle.manageddataaccess.client/version/settings/setting[@name='TNS_ADMIN']";
var attrs = xmlDoc.SelectSingleNode(path).Attributes["value"].Value = "some value";
xmlDoc.Save(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
ConfigurationManager.RefreshSection(path);

System.Configuration.Configuration cnf = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
    cnf.AppSettings.Settings["TNS_ADMIN"].Value = "my value";
    cnf.Save(ConfigurationSaveMode.Modified);

config.AppSettings.Settings.Add("TNS_ADMIN", @"anylocation");

config.AppSettings.Settings["TNS_ADMIN"].Value = "NewValue";

<appSettings>
    <version number="*">
        <settings>
            <setting name="TNS_ADMIN" value="asd" />
        </settings>
    </version>
</appSettings>