Note this may just be down to your chosen media player ignoring the rotation flag in the video.
If you don't mind using a command prompt then download FFMPEG from here:
https://ffmpeg.zeranoe.com/builds/
E.g. for 64-bit Win10 choosing 4.2.1 (or higher) 64-bit and static (chosen options are darker) then "Download Build".
From the Zip file you just need ffmpeg.exe from the bin directory. If you put it in the same directory as the file you may need to type ".\ffmpeg" not "ffmpeg" to run it, as not all Windows installations have the current directory in the search path for programs.
Example commands:
(1) Rotate losslessly, which requires a player that supports the rotation flag (VLC and MPC-HC don't, Windows Media Player does - but it might be correct already if you haven't tested with WMP):
ffmpeg -i input.mov -metadata:s:v:0 rotate="90" -codec copy output.mov
Or rotate="270" to go the other way (-90 used to be unreliable). Note this is absolute, so if it was 90 already setting 90 will not change it, "ffmpeg input.mov" will show it in the output.
It took about one second for me.
(2) Rotate and scale, re-encoding:
ffmpeg -i Input.mov -vcodec libx264 -acodec copy -s 640x360 -vf "transpose=2" output.mov
For the transpose parameter you can pass:
0 = 90CounterCLockwise and Vertical Flip (default)
1 = 90Clockwise
2 = 90CounterClockwise
3 = 90Clockwise and Vertical Flip
Use -vf "transpose=2,transpose=2" for 180 degrees.
If you don't want to scale leave " -s 640x360" out.
