Upgrading from Android 5 to 6, or 6 to 7, can leave residual cache and app conflicts. A factory reset cleans the system.
A correct firmware update is free, safe, and reversible only by factory reset. A wrong firmware is irreversible damage. Always double-check your model number. sony kd-43x8309c firmware
Before updating, you need to know what is currently installed. Upgrading from Android 5 to 6, or 6