Sometimes firmware is hosted on obscure FTP servers or community forums (like XDA Developers). If the official site is outdated, advanced users check the "OTA Link" found in the Service Menu. Warning: Accessing the Service Menu and changing settings can brick your TV.
Even with a perfect guide, things go wrong. Here is how to fix them. Tcl Tv Software Update Usb
If the official site fails, two reputable communities exist: Sometimes firmware is hosted on obscure FTP servers
Crucial Note: TCL uses two primary file types. You need the correct one for the USB method: Follow on-screen instructions