Simple, just adjust your pledge in the pledge manager.
And if you want, say the Refinery Foreman but also a Grunion Mucker level, simply select Refinery Foreman and add a dollar amount that covers the Grunion Mucker level. For instance, if you want a Refinery Foreman and two Grunion Muckers, pledge for $78 ($38 + $20 + 20).
Don't see the answer to your question? Ask the project creator directly.