Without knowing the context, "tune down" is something you can do with a musical instrument whose design allows changing the tuning to a different reference pitch. To tune down the instrument, you change the tuning to a reference pitch that is lower than the customary or standard reference pitch for that instrument. This action is done more frequently with stringed instruments such as guitar or violin/fiddle than with wind and brass instruments.

I think you mean "tone it down", based on your other comment here.

I am sorry there is no such collocation , idiom or phrase I ever heard like "Tune Down" , do you mean "Tone Down" ? There are phrases like tune up , tune in and tune out. (Used for adjusting the TV channels , radio channels or musical instrument tune adjustments). https://www.collinsdictionary.com/spellcheck/english?q=tune+down https://www.oxfordlearnersdictionaries.com/spellcheck/english/?q=tune+down https://www.merriam-webster.com/dictionary/tune%20down https://www.dictionary.com/misspelling?term=tune%20down&s=t I mentioned the top 4 best dictionaries links you can check for the definition , examples and similar words.