Decide whether the problem is sideways or mirrored
A sideways picture and a reflected picture need different transformations. Before editing, inspect a recognizable object or readable text in the authorized local clip. Keep an unmodified original so the result can be compared without relying on memory.
For an original orientation check, readable writing can help distinguish a mirror image from a quarter-turn. Do not decide from an almost symmetrical scene. If the confusion exists only on one external display, compare the same file on the Mac before editing the media itself.
Use the documented selected-clip route
Apple’s QuickTime guide uses View, Show Clips, then selection of the intended clip, outlined in yellow. The Edit menu offers rotate left or right and horizontal or vertical flip. Choose the transformation that matches the identified problem, then Done.
Inspect the selected segment before applying the action. In a multi-clip movie, the surrounding segments may already be correct. Apply one transformation, review it and undo a mistaken choice before stacking more rotations or reflections.
Review and save a distinct result
Compare the corrected segment with the original reference and inspect neighbouring clips. Check that the subject is upright and text has the intended direction. A picture filling the window is not itself an orientation check.
Save a distinguishable edited copy and reopen it before replacing any reference used elsewhere. If the source is still displayed incorrectly only through one viewing path, investigate that path rather than repeatedly transforming the file. No media was edited for this guide.
Questions answered
Is flipping the same operation as rotating?
No. Choose between changing orientation and reflecting the picture according to the actual visual problem.
Why inspect the yellow selection first?
Apple’s route applies the edit to the selected clip. Confirm the intended segment before changing a multi-clip movie.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.