toggle through info screen to "lake temp"
get up to riding speed, 24, 25 on gps
press and hold the DOWN arrow for 3 seconds and the display will start blinking.
Use the up and down arrows to adjust your digital speed (on right guage) to match gps. I forget what button to push when done, I think it just times out after a few seconds if you do not touch anything.
Test it at faster speeds and again at slow surf speeds.