I used this because I couldn't find anything to edit metadata for an mkv file encoded with x.265. Subler (as of current 1.09 version) can't handle x265, and is my usual 'go to' tool, also Subler can only remux and save to MP4 container.
MKVToolnix worked great for editing metadata, and saved back to original MKV container, and handled x265 video track just fine. While I was at it, I removed foreign language audio and subtitle tracks that I didn't need, and reduced the size of the file by 25%.