If the download site you selected to download it from doesn't tell how to use what you download it's a rather bad site. Why not find a better one?
Anyway, https://www.macworld.co.uk/how-to/iphone/how-set-song-iphone-ringtone-3635061/ tells how to do it.
Hello Everyone!!!! I'm new here so please bear with me. I used to have the iPhone and switched to the Samsung Galaxy S7 about a year ago. I have decided to switch back to the iPhone and I need everyone's help. I want to add a custom ringtone to the iPhone and I'm not sure how to do that. Does anyone know?
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