If you need to convert an MKV file to MP4, VLC Media Player makes it easy. Here’s how to do it:
Open VLC and go to 'Media' in the top menu.
Select 'Convert/Save.'
Click 'Add' to find and select your MKV file.
After adding the file, click 'Convert/Save' again.
Choose where you want to save the new file and rename it if needed.
Next, select your desired format – I recommend 'Video - H.264 + MP3 (MP4),' which works well for uploading to YouTube.
Finally, press 'Start,' and VLC will convert your file. Your new MP4 will appear in the folder you selected.
Get VLC:
https://www.videolan.org/vlc/
Note that both your MP4 file and the original MKV file will be available to you. The conversion does not remove the MKV file.