public void ChangeVersion( 
   VersionBase fromVersionBase,
   VersionBase toVersionBase
)

public void ChangeVersions(Geodatabase geodatabase, string toVersionName)
{
  using (VersionManager versionManager = geodatabase.GetVersionManager())
  {
    VersionBaseType versionBaseType = versionManager.GetCurrentVersionBaseType();

    if (versionBaseType == VersionBaseType.Version)
    {
      Version fromVersion = versionManager.GetCurrentVersion();
      Version toVersion = versionManager.GetVersion(toVersionName);

      // Switch between versions
      MapView.Active.Map.ChangeVersion(fromVersion, toVersion);
    }

    if (versionBaseType == VersionBaseType.HistoricalVersion)
    {
      HistoricalVersion fromHistoricalVersion = versionManager.GetCurrentHistoricalVersion();
      HistoricalVersion toHistoricalVersion = versionManager.GetHistoricalVersion(toVersionName);

      // Switch between historical versions
      MapView.Active.Map.ChangeVersion(fromHistoricalVersion, toHistoricalVersion);
    }

    // Switch from HistoricalVersion to Version and vice-versa 
    // MapView.Active.Map.ChangeVersion(fromHistoricalVersion, toVersion);
    // MapView.Active.Map.ChangeVersion(fromVersion, toHistoricalVersion);

  }
}

await QueuedTask.Run(() =>
{
  //Getting the current version name from the first feature layer of the map
  FeatureLayer flyr = MapView.Active.Map.GetLayersAsFlattenedList()
    .OfType<FeatureLayer>().FirstOrDefault();  //first feature layer
  Datastore dataStore = flyr.GetFeatureClass().GetDatastore();  //getting datasource
  Geodatabase geodatabase = dataStore as Geodatabase; //casting to Geodatabase
  if (geodatabase == null)
    return;

  VersionManager versionManager = geodatabase.GetVersionManager();
  ArcGIS.Core.Data.Version currentVersion = versionManager.GetCurrentVersion();

  //Getting all available versions except the current one
  IEnumerable<ArcGIS.Core.Data.Version> versions = versionManager.GetVersions()
    .Where(v => !v.GetName().Equals(currentVersion.GetName(), StringComparison.CurrentCultureIgnoreCase));

  //Assuming there is at least one other version we pick the first one from the list
  ArcGIS.Core.Data.Version toVersion = versions.FirstOrDefault();
  if (toVersion != null)
  {
    //Changing version
    MapView.Active.Map.ChangeVersion(currentVersion, toVersion);
  }
});