If all you want to do is read the video file and encrypt it, then just read it as a binary file, excrypt it, and write it back.
If you want to encrypt individual frames so the the video shows as rubbish in a normal media player, then you have to extract each frame, encrypt it, and then re-build the video file. Be aware though of two things:
1) The resulting file is likely to be very large!
2) The chances of decrypting the frames again is extremely poor. Video storage uses a lossy compression technique which will probably destroy your encrypted data.
You can find the info to extract the frames here:
Extract Frames from Video Files[
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