Launch iMovie by clicking its icon in the dock.
Select Import Movies... from the File menu.
If prompted select the video format you would like to use and then click the Ok button. Note: In most cases it is fine to use the Large video format.
Select the movie you would like to import from the popup window. Then choose to either Add to existing Event or Create new Event. I created a new event and called it Upside Down Video. Now click the Import button.
You will now notice your new Event added in the Event Library. Select the event.
Select the portion of video you would like to work with. You do this by clicking at the start point and dragging your mouse to the end point, then release. Notice a yellow box will outline the clip you selected. To fine tune your selection you can use the edges of the yellow box as handles. Drag these handles to fine tune your selection.
Drag your selection to the empty filmstrip above.
Select the clip you just added to the filmstrip.
Click the Crop Tool on the toolbar.
Notice the preview window now shows your video clip with some controls overlaid. To rotate your video clip you simply click the left or right arrow to rotate.
Once the video clip is oriented correctly, click the Done button.
Now select Export using QuickTime... from the Share menu. NOTE***: You could also use any of the other Share methods and your output will be your newly rotated video clip.
Give the movie a name using the Save As field then click the Save button. Your exported movie will now be rotated correctly!