Tails includes an automatic mechanism to upgrade a USB stick to the latest version of Tails. When an automatic upgrade is performed, only an *upgrade package* is downloaded. The upgrade package contains all the changes made to Tails since Tails was first installed or was last manually upgraded.
After starting Tails and connecting to Tor, *Tails Upgrader* automatically checks if upgrades are available and then proposes you to upgrade your USB stick. The upgrades are checked for and downloaded through Tor.
You want to upgrade by cloning from another TailsUSB stick which is already up-to-date, for example, when working offline or with a slow Internet connection.