Here is the download page for flash related software (currently R24.2.1):
https://developer.nvidia.com/embedded/linux-tegra
If you download JetPack, then JetPack will handle most downloads and setup for many extras. This requires your desktop host to have Ubuntu 14.04 on it. This has instructions with it.
If you just want to flash, then your desktop host can be any 64-bit Linux. In that case you’d download the “driver package” plus the “sample rootfs”.
In the case of just flash, you’d unpack the driver package. This produces subdirectory “Linux_for_Tegra”. Within that is subdirectory “rootfs”. You’d use sudo to unpack the sample rootfs into that directory. Once sample rootfs is there, cd back to the “Linux_for_Tegra” directory and use sudo to run “./apply_binaries.sh”. At that point you can put your Jetson in recovery mode (hold down the recovery button while tapping power up or power reset). Be sure to use the supplied micro-B USB cable for this. To verify recovery mode your host will have output from the command “lsusb -d 0955:7721”.
To start the flash, use sudo:
sudo ./flash.sh -S 14580MiB jetson-tx1 mmcblk0p1
Note that this takes time, and you should probably have about 25GB or more of free disk on your host before starting flash.sh.