You can either set the book to free on some other sites and hope that Amazon price matches, or you can join the KDP Select program and offer the book for free five out of every ninety days. The Select program is described here: https://kdp.amazon.com/self-publishing/KDPSelect
Once you start your promo, you will want to submit it to a variety of web sites such as PixelofInk.com - but be sure to do so at least a week in advance. If you can get on Pixel of Ink you'll get a ton of downloads.
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