The factory trans temp sending unit is located in the solenoid body in the transmission pan. It is in the front, located just to the right or left of center. Two wires go into a black plastic housing.
It is a 1/4 turn type sensor. Remove the plug, give the sending unit a quarter turn and pull it straight down. Reinstallation is the opposite.
If you're going to replace yours, I'd recommend looking at your new one before disconnecting anything. Then match it up with the one in your transmission.