Article in English: You can only rotate external displays, and the display must support rotations (not all do). If you are running older versions of OS X, you can attempt to force the built-in display to rotate, but this does not work in newer versions. This will show all of your connected displays Select your external display from the available displays. If you want to try to rotate an internal display, such as on a MacBook or iMac, see Step 6. You can select 90°, 180°, or 270°. These represent how many degrees the display will rotate to the right. If all of your screens rotate when you enable rotation on one of them, mirroring may have been enabled. This makes all of the screens mimic each other. Click the "Arrangement" tab and uncheck the "Mirror Displays" box. If you are using Mavericks or earlier, you can attempt to force a built-in display to rotate by opening a special version of the Displays menu. Make sure System Preferences is closed before proceeding. Do not attempt this in OS X 10.10 (Yosemite) or later, as it can result in a serious error.  Click the Apple menu and select System Preferences. Press and hold ⌘ Cmd+⌥ Opt and click the "Displays" option. Your built-in display should have a rotation menu available.

Summary in Vietnamese:
Nhấp vào trình đơn Apple và chọn "System Preferences" (Bảng điều khiển hệ thống). Kích vào tùy chọn "Display". Chọn Màn hình ngoài. Cài đặt trình đơn "Rotation" thành tùy chọn mà bạn muốn trong gờ "Display". Đảm bảo không kích hoạt tính năng Mirroring (Phản chiếu). Thử xoay màn hình tích hợp (OS 10.9 và thấp hơn).