How do you manually move an electric seat back and forth in a classic 91 Range Rover switch is broken?

You can't. You have to patch a 12V wire to one or more of the seat motors to make it move. Obviously it would be good to use a temporary or switched supply of 12V. If your driver's seat has stopped moving it is likely to be either the switch itself, or the memory control unit (ECU). This smart toy remembers where you like to have your seat, and also where you like to have your mirrors. They die and cost many $$$ to replace. Simply bypass the smart box and the seat will move with the switches as normal -- it just won't remember where you like to sit. If you can't remember that for yourself, maybe yhou should spend the $$$ on the spare part.