at the bottom of the screen, there is a toolbar. Next to the letter "A", it has the degree of rotation. Just tell it how much you want the selected part to rotate. Next to that is your X and Y mirror. this is a quick and easy way to flip and mirror parts 180 degrees.
EDIT: you can also use your arrow keys to slowly change the angle. Make sure while you are nesting you click on the part you want to move. You can drag a box around all of them if you want to rotate them all.