You download the iso, flash the iso to an USB install stick using etcher or Rufus, stick the USB stick in your Truenas box, start the box, choose to boot from USB and then follow the installer
Edit: Here’s the official documentation
You download the iso, flash the iso to an USB install stick using etcher or Rufus, stick the USB stick in your Truenas box, start the box, choose to boot from USB and then follow the installer
Edit: Here’s the official documentation