You can update via USB flash drive. This works for both upgrades and downgrades (downgrades require a forced IMG flash).
Go to Settings > Device Preferences > About > Contact Us . Look for the "Software Version" (e.g., V8-R51MT05-LF1V...). The R51 indicates the RT51 platform. Firmware Update Methods 1. OTA (Over-the-Air) Update Firmware TCL 50EP660
: Firmware updates often include better calibration for HDR and local dimming, significantly improving picture quality for movies and games. You can update via USB flash drive
Copy the .zip file (do not extract it) to the root directory of the USB drive. Insert the USB into the TV's USB 2.0 port. Firmware TCL 50EP660