how to change page orientation from portrait to landscape in an existing PDF ?

If you want to transition content from a page that is currently Landscape to Portrait orientation (or from Portrait to Landscape), rather than rotate text while maintaining the current orientation, one approach is to select the Edit tool in the current document and Copy the content you wish to re-orient; then create a new blank document in the desired orientation, select the Edit tool, and Paste the copied content into the new document.  This may require copying and pasting content in sections and/or resizing the content to fit the new page.  This may not work with all content, but has worked for me when I have needed to change orientation.  

Sorry, but I believe you did not answer the question properly.  The question is:  "How to change page orientation?"  The instructions you provided are for ROTATING the document or object--that is a different thing.

To change orientation, the page will be flipped--but the writing should still be running from left to right (in English).  Your rotation method will make the document difficult to read for most people, because the words will be at a 90-degree angle.