In order to rotate multiple photos, you need to open the Organizer ("Organize" >> "Organizer"). In the right-side content panel, you need to select the photos you wish to rotate. You can use the Shift and Ctrl keys for multiple selection. Once you have selected the photos, choose the appropriate option from among the in the "Selected" >> "Rotate" tools (2).
What can I do next? You may want to take a look at the variety of tools that are accessible through the Organizer.