// in order to find a dockpane you need to know it's DAML id
var pane = FrameworkApplication.DockPaneManager.Find("esri_core_contentsDockPane");
// get the undo stack
OperationManager manager = pane.OperationManager;
if (manager != null)
{
// undo an operation
if (manager.CanUndo)
await manager.UndoAsync();
// redo an operation
if (manager.CanRedo)
await manager.RedoAsync();
// clear the undo and redo stack of operations of a particular category
manager.ClearUndoCategory("Some category");
manager.ClearRedoCategory("Some category");
}