That depends on what you mean by "back up". Read Backing up an Android Phone. Any file you can back up to a computer you can copy to a Kindle too. How? The same way you copy any file. Either plug them both into a computer at the same time and copy from one to the other or use something like WiFi File Transfer on one and a web browser on the other. (They both have to be connected to the same router by wifi.) There are also Bluetooth file transfer apps.