public void ChangeVersion( 
   Version fromVersion,
   Version toVersion
)

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);
  }
});