hello kunice,
after you finish the installation through JetPack, it would download the generated image and necessary tools under your JetPack folder.
for example, you should found the flash.sh at below path.
JetPack/3.0/64_TX2/Linux_for_Tegra_tx2
you could put the device in forced-recovery mode and using below commands to flash the image with existing system.img for TX2
sudo ./flash.sh -r jetson-tx2 mmcblk0p1