If you want to change the container of a video file (no transcoding), you can use FFmpeg and the -c copy command.
Keep in mind I keep FFmpeg exes in a folder called PATH_Programs, so I put the video file in the same folder.
Let's say you want to change video01.mkv to video01.webm. I would open the Command Prompt (with Administrator Rights) and type the following:
C:\PATH_Programs>ffmpeg -i video01.mkv -c copy video01.webm
FFmpeg will just change the container from Mkv to Webm.
