What version of the Mayfly board do you have? The newer boards have a different regulator than earlier versions, so make sure you’re looking at the right datasheet and resistors.
If you’re comfortable removing and reinstalling SMT resistors, then it is possible to remove that resistor and replace it with another one to achieve different output voltages. But you have to be careful with supplying 12v to external devices so that you don’t exceed the maximum safe current and that the return data is at a safe voltage, otherwise you’ll damage the Mayfly. It’s likely you might need some external level shifters on the return data lines, which is why I didn’t include 9v and 12v excitation options on the Mayfly to begin with. Things can quickly go bad if users aren’t careful about voltage levels, so I decided to keep higher voltage stuff off the Mayfly. You can add an external boost regulator for just a few dollars each (5, 9 or 12v) from here: https://www.pololu.com/product/2117
Then you can put that boost regulator on a Grove protoshield (https://www.robotmesh.com/grove-protoshield) along with a level-shift breakout and then a header or screw terminal, and then you’ll have everything you need for safely powering an external device with a different voltage.