You take a screw driver and open your SNES Myth. There are 2 screws on the back, and 2 on the front (hidden under the label).
Then you take your Nero Dual Programmer and one of the flat cables that came with it. You should've received 2 cables; one with 8 pins and one with lots of pins (20 or 34 or whatever the number is). IIRC the one with 8 pins is for the N64 Myth, while the one with more pins is for the SNES/MD Myth.
So you plug one end of the cable into the Nero. There are two slots on it; you should use the "upper" slot, the one on the same side as the logo. And the text on the cable should also be facing up. Pull the plastic "plug" covering the slot out a bit when inserting the cable, the push it back in to secure the cable.
Then plug the other end into the Myth, plug the Nero into your computer, run the updater program and press the button it, and wait. If the update fails, check the cable connections and try again.