I used Windows Media Player on my PC to slow down the music. 
- Play a file, and then switch to Now Playing mode: click the Switch to Now Playing button
in the lower-right corner of the Player.
- Right-click an open space in the Player (e.g., to the left of the Stop button) , point to
Enhancements, and then click Play speed settings.
- Move the Play Speed slider to the speed at which you want to play the file, or click the Slow,
Normal, or Fast links.
Note: Slow Normal and Fast are preset speeds. Normal is 1.0 (default), Slow is 0.5, and Fast is
1.4.
However, Windows Media Player can only access common format audio like mp3, not Spotify music. 
Therefore I used AudFun Spotify playlist converter to convert Spotify music to mp3, flac, wav etc.
It helps me a lot. It’s incredibly easy to use. 