How to update Smart Slider 2 to Smart Slider 3?

Smart Slider 2 and Smart Slider 3 are two different extensions/plugins, so you can simply install Smart Slider 3 on your website which has Smart Slider 2 and they will be just next to each other.

You can't update Smart Slider 2 to Smart Slider 3, and you will have to create new sliders, if you want to have Smart Slider 3 sliders in your website. In the future Smart Slider 3 will be developed further, and Smart Slider 2 will be kept updated for as long as it's necessary.