Turn off the TV, insert the USB, and then power it on. Some models require holding the power button or navigating to Settings > Support > System Update > Update via USB .
If official support is unavailable for this older model, technicians often use community-driven databases. Note that these files are often split into parts (like "part2.rar") and may require a paid account or specific community credits: Download MS6486T PB753 1920x1080 Hisense 43N2170 part2 rar
Select both parts and extract them to get the main software file (usually ending in .bin or .pkg ). Turn off the TV, insert the USB, and then power it on