not sure on your unit specifically, but my SR Mariner unit has a small rubber plug on the back that I can pull out and fit in a little eyeglass screwdriver, there is a calibration screw in there that you turn for the adjustment.
I just recalibrated mine a few weeks ago because I noticed it seemed to be off a little over half a knot. It was real nice last week going through the east river knowing that it was properly calibrated, I was able to know how much speed was current vs actual boat speed.
Saved me a bit of fuel too when I had to motor sail up the hudson to the mooring against the current, I saw the hull speed was about 6kts, but GPS speed was about 2kts. I probably would have given it more throttle with no actual speed gain if I hadn't known I was already going 6kts through the water...
