Editing metadata of H.264 MP4 videos

Solution 1:

Most of the fields can be filled in using MP4Box as follows:

MP4Box -itags writer=me file.mp4

You can have a list of supported tags with MP4Box -tag-list. If some fields are not supported, file an issue.

Solution 2:

As of version 11.39, exiftool has the ability to edit tags in MP4/Mov files. To edit the tags listed in this question, the command would be

exiftool -Title="New Title" -Description="New Description" -Comment="New Comment" file.mp4

This command creates backup files. Add -overwrite_original to suppress the creation of backup files. Add -r to recurse into subdirectories.

See exiftool's Quicktime Tag page for a list of the tags that exiftool can edit.