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There’s nothing to unsolder. Solder Jumper SJ25 has 2 small pads. By default, they are separated and not connected at all, and that makes the on-board boost circuitry output 12 volts. By putting a solder blob on SJ25 and connecting the 2 pads, you add some extra resistance to the circuitry that controls the voltage and it’ll output 9 volts instead. So there’s no chance you can do it wrong, as long as you’re adding solder only to SJ25. Once done, you can verify your success by putting a voltmeter on the SW12 pin of the headers and when the switched power output is on (as indicated by the red LED in the lower left corner of the Mayfly), you should see 9v there, and on any Grove jack Vcc pin if you adjusted the pin jumper next to that particular Grove jack (as demonstrated about halfway down on the Jumper Settings page.)