First, plug in the USB to the Xbox. Then, go to the Xbox dashboard, then go to "settings"; then to "storage'; then select your hard drive (where your game is currently); then to "games & apps"; then, "Minecraft"; then look down the list for your world name; then press A to open the menu box for that world; then "copy" (which will leave the original on the hard drive and make a copy of it on the USB).
A few cautions - DO NOT rename the copy on the USB. DO NOT try to rename the original and then copy the copy back onto the hard drive. These will generally cause either a corruption error or the world on the hard drive to get deleted.
A few cautions - DO NOT rename the copy on the USB. DO NOT try to rename the original and then copy the copy back onto the hard drive. These will generally cause either a corruption error or the world on the hard drive to get deleted.