First you need to identify the version of your software. In order to do this you need to go to Menu, then Settings, then Device Information, then based on the version found please go to the official support website and download the firmware file according to the version installed. Transfer that file to Kindle and follow the instructions from the linked word.
Basically, you need to go into Menu > Settings > Menu again > Update your Kindle. Perform the update if it's available and wait for the device to restart and that's it.