Or little computing tricks and hacks
During the my daughter’s music camp, I recoded some videos rotated 90°. VLC offers an option for rotating but just for viewing. Instructions here.
To permanently rotating the video use the powerful ffmpeg:
ffmpeg -i input-file -vcodec libx264 -preset medium -crf 24 -threads 0 -vf transpose=1 -acodec copy output-file.mkv
Taken from the Ubuntu Forums.
The output is in the Matroska Multimedia Container file format (.mkv) and you can find an explanation in Wikipedia.