how to convert an MKV file to MP4
to convert an MKV file to MP4:
Run "VLC media player".
From main menu select "Media" then"Convert / Save...".
On "File" tab, click "Add..." button.
Choose you MKV file. After that, it should be added to the list. (Alternatively, you can drag&drop a file into the list.)
Click "Convert/Save" button.
Select Profile: "Video - H.264 + MP3 (MP4)".
Click on the icon button that has hint: "Edit selected profile".
On "Encapsulation" tab, leave "MP4/MOV" selected.
On "Video codec" tab, check both "Video" and "Keep original video track".
On "Audio codec" tab, similarly select both "Audio" and "Keep original audio track".
Click "Save" button.
In "Destination" selection, click "Browse" button to select directory and file name for you destination MP4 file.
Click "Start" button.