Enter Ctrl-Alt + up-arrow (that is, hold down both the Ctrl and Alt keys, and type the up-arrow key (the topmost in a bank of four arrowed keys)). Then release the Ctrl and Alt keys. After a moment or two your display should revert to the normal way up.
On some versions the key sequence Ctrl + Alt Gr + up-arrow can be used instead.
You can switch the display to 'upside-downness' by doing Ctrl-Alt + down-arrow (or Ctrl + Alt Gr + down-arrow).
Not all display drivers support this function, but in such cases your display should never go 'upside down' anyway.
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This is question number 1476, which appears in the following categories: