This is the most common method for 3RT841B boards when the TV cannot connect to the internet or won't boot past the logo.
If your 3RT841B is a purely mechanical contactor (no electronics), it has no firmware. This guide applies only to the electronic variants – check your exact part number suffix (e.g., 3RT841B-1A... or 3RT841B-2B...). 3rt841b firmware upd