http://biblemeowimkh3utujmhm6oh2oeb3ubjw2lpgeq3lahrfr2l6ev6zgyd.onion/content/bible/operating-systems/tails/backing_up/index.html
On the Welcome Screen, enable the Administration Password. You can make it whatever you want it will reset after you shutdown tails. (Click the + in the bottom left) Once you have booted up tails with the Administration Password enabled go to, Applications-> Accessories-> Files to open the Files browser Plug in your backup USB stick.