You may rotate image with a few different ways.
For simple 90 degrees rotation please use four big buttons on the top.
For a fine rotation, please use two round Rotation Handles near the right top corned of the cropping frame.
As an alternative you may input rotation degrees number (up to one hundredth of degrees) in the appropriate text field.
You may easy repeat last used rotation. The program remembers last applied angle. To repeat rotation simple select Last Rotation from Edit menu.