Once you have the unprotected file, follow the instructions for your specific device. For Android Devices

Go to Settings > Sound & Vibration > Phone Ringtone .

You cannot use tracks from Amazon Music Prime or Unlimited directly as ringtones because they are encrypted. You have two main options to get a usable file:

Setting a song from Amazon Music as your ringtone isn't a direct process because standard streaming downloads are and locked within the app. To use your favorite tracks as a ringtone, you must either purchase the MP3 directly or use a conversion tool to unlock the file. Step 1: Obtain an Unprotected MP3 File