If you measure 3.3v on the Sw_5v pin, then unfortunately you have a board with the incorrect regulator. There’s no way to get your board to generate 5v unless you replace the regulator. While it is possible to replace it if you have the right tools and you have experience removing and replacing a SOT-23 surface mount device. However, we have new boards with the correct part that we have shipped to Amazon to replace the existing inventory. I will also be contacting each customer directly through Amazon to tell them how they can exchange any affected boards for a new one.
Keep in mind that most users will not need to use the 5v setting on the Grove connector jumper. The Mayfly is not tolerant of 5v I/O signals, so any peripheral devices you connect must not send any voltage greater than 3.3v back to the Mayfly I/O pins. The 5v options is only there for experienced users who can ensure that they have proper level-shifting circuitry or other protective measures to ensure that the Mayfly only sees 3.3v signals. In my case, I use several analog sensors that need to be powered by 5v supply, but only output a 0-2.5v signal. Other digital sensors that require 5v will be connected to a level-shifting adapter board via the Grove connector.