1. Go to the last page you want to keep current page orientation (i.e Portrait) and go to Page Layout |Breaks | Next Page.
2. Now click anywhere in the page where you want to change the page orientation, in this case Landscape, and go to Page Layout | Orientation and select Landscape
If you want to change the orientation for only one page and leave the rest as they were you have to do #1 and #2 just after you changed the orientation.