Tap the download button. The MP3 saves to your Downloads folder.
Go to Settings > Sound & vibration > Phone ringtone (or Ringtone).
Tap the + icon or 'Add ringtone', then browse to your Downloads folder and select the MP3 file.
You can also move MP3 files to the 'Ringtones' folder on your device. They will automatically appear in your ringtone list.
Looking for device-specific instructions? Check these official guides: