How to put another video's voice over my video?

I have a project to do where I have to take pictures of nature and make a slide, but I have this piano piece on my video on my iPhone and I want it to play with the slide i'm gonna make, please tell me how I can make that happen?

So don't take my word for it because I'm not really great at all this.

I think you need to convert the piano piece into an mp3 file, then use it as a song on your video.

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