Making this modification is a bit tricky, since it's mostly a mechanical problem of identifying the ends of the windings. Note that on my generator, each winding had two wires, so there are a total of 4 wires, but each pair are soldered together to create two windings.
In the top drawing below, the basic generator consists of two windings, each delivering 115 volts. In order to provide 240 volts (the second drawing), the windings are connected in series via the green wire (that's a notional green wire. In practice the ends of the windings are close enough together that they can be directly connected).
To double the current at 115 volts, you need to put the windings in parallel (third drawing). You will need to study the wires until you can identify the two windings and the connection between them. Sever that connection and reconnect the windings in parallel. If you screw up and connect the windings anti-parallel, eg, backwards, you'll get zero output and the generator will either blow the breakers or will overheat very quickly (but not instantaneously, so you will have 10 or 15 seconds to figure it out).