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If it is a brushless motor, simply changing the voltage and amperage has a similar effect to changing the gear ratio.
(high amps low volts is like "low gear" and low amps high volts is like "high gear"). This can be done without changing the actual power (wattage... watts = amps times volts) of the motor.
I'm sure it's possible to change the actual gears used by a geared hubmotor, but I suspect it will cost you more money than buying a motor/controller/battery that has the amount of torque&speed you want in the first place. If you can obtain the services of a skilled machinist and machine shop for free (or almost free) then maybe changing the gears would be the way to go, though.
Another way to change the effective "gear ratio" of a hubmotor is to change the size of the rim&tire.