Open the Alexa app on your phone and press the Menu button ( ) and go to » Settings» > » Music and multimedia». If you haven’t connected your Spotify and Echo accounts yet, click » Link Account» on Spotify.com and sign in with your Spotify credentials.
Once your accounts are linked, click » Select default music services», and you can select Spotify as your default music library. This means that when you ask Alexa to play any song, it will default to Spotify. This will only work for Spotify Premium members.