There are two ways to factory reset your TCL Fire TV, through settings menu, and in case you cannot access your settings, the second way is via the remote, read below to explore both options:
Via Settings
When you are on your home page, navigate to Settings - Device & Software - Reset to Factory Defaults.
Settings
Device & Software
Reset to Factory Defaults
Reset to Factory Defaults Screen
Via the Remote
To reset your TCL Fire TV to its factory settings can be done via your remote control. To do this, hold the Back button and the right side of the navigation circle, at the same time, for 10 seconds. See image below:
Note: If you see your on-screen cursor moves rapidly across the screen, this is ok.
Once done you will see a screen similar to the one below. This will give you the option to continue with the reset, or to cancel the reset. If nothing is selected after 20 seconds, it will continue with the factory reset. See image bellow:
Note: By factory resetting your TV, all of your settings, passwords, installed apps will be erased.