Software updates for your Kindle will automatically download and install when it is connected wirelessly. If for some reason they will not install you can install them manually.
Find your device version and download the appropriate update from https://www.amazon.com/gp/help/customer/display.html?nodeId=GKMQC26VQQMM8XSW
This page explains how to get the update on to your Kindle. https://www.amazon.com/gp/help/customer/display.html?nodeId=GFAE5G5UGCYA25DW