As the previous post, you can drag and drop or you can use Calibre as well to send to device for a Kindle (my preference). If the format you have is not one Kindle recognises, i.e. EPUB, Calibre will offer to do the conversion for you, then it sends the MOBI file.
If you have already done the conversion and have two copies, 1 EPUB and 1 MOBI, Calibre will either just send the MOBI, or will prompt you to pick the format to send (there is a setting in preferences somewhere I think).
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