It isn’t hard. Physically turn the battery around (you might need longer and, depending on the terminal type, different leads; the Positive terminal is larger than the Negative). Reverse the low tension connections on the coil, so that the cable from the distributor connects to the negative terminal. Now take a lead from the battery Positive terminal and touch it briefly to the small terminal on the back of the dynamo. You will get a spark when you do this, but it repolarises the dynamo to the new setting.
And that’s it!
