how can I transfer a song from mp4 to mp3 on the 2017 Mac book pro

hi I have a new 2017 MacBook Pro I bought a song of iTunes and trimmed it with I movie. it is under as an mp4 however I need it to be an mp3. The instructions I have read previously have only gotten me so far and doesn't give me the "create mp3 version" option.

however these last instructions were from 2012 and I'm wondering if there is an updated new way?

thanks.