This[0] is a guide on exactly how to do it, but basically you install a Calibre extension[1], get your Kindle books (Calibre can automatically pull them from your Kindle, or you can download from Amazon.com), and import them. The plugin will strip the DRM as they are imported into Calibre.

[0]: https://www.howtogeek.com/162994/how-to-strip-the-drm-from-y...

[1]: https://apprenticealf.wordpress.com/2012/09/10/drm-removal-t...