Correct the axle ratio in a 94 k1500 4x4

Hi:
I have a Blazer K-1500, 4x4, 350CI TBI. It has a funny problem: when speedometer indicates 50MPH, Im going 60MPH, when indicates 75MPH is going at 90MPH.
I dont know if it is possible to change the information in computer so I can have dependable readings in speedometer and odometer, as well as stop having problems with police.
Thanjks Oscar
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