Powerglide shifts too soon: '66 SS

I have a '66 SS. 396, 360HP, 2 speed powerglide. I am not certain of the rear end ratio (probably whatever was stock).
It shifts at approx 23 mph. This seems very soon. When should it shift? A friend said to adjust the modulator. I have attempted to do so, but have been unable to. Maybe I am trying wrong. Is this correct advice? Could the problem be that the modulator is malfunctioning all together and needs replacement? Where should I go from here?

Thanks,

gerald