Open VLC Media Player
Open the 'Media' Menu and select 'Covert/Save'..
Click the +Add.. button and navigate to your source video.
With your Video loaded, select the drop down arrow next to the convert/Save button
From the menu select 'Convert'
On the next window, select a profile. This is the format you want the output file to be.
usually the default 'Video - H.265 + MP3 (MP4)' will be sufficient.
Finally select a destination folder where you wish the file to be written, and give it a name.
Select Start and let it play through.