After the update completes, your TV will power cycle. Do not insert the USB again yet.
Always double-check your model.
A common search is for the Toshiba Fire TV 55LF621U21 getting stuck on the Amazon logo. In this scenario, an OTA (Over-the-air) update failed due to a power outage. The verified fix was: toshiba tv firmware update usb verified
This is why verification matters. A random file from a forum would have had a different hardware ID and failed permanently. After the update completes, your TV will power cycle