how to make my arduino uno R3 to be 2v(1023/2v) only instead of 5v(1023.5v)??
The Arduino Uno microcontroller chip will run on any voltage between 1.8V and 5.5V, with some limitations. Study this low power operation tutorial carefully.
If you are referring to measuring voltages using the ADC, see this page.
Use a suitable resistor between the 3.3volt pin and the Aref pin, and set Aref to EXTERNAL in setup().
analogReference(EXTERNAL);
Might have to experiment a bit with the value, but 22k should get you in the ballpark.
Other option is to set Aref to 1.1volt (INTERNAL), and use a 1:1 voltage divider on the signal you want to measure.
Assuming it is a 0-2volt voltage (not ratiometric) source you want to measure.
But we might have set you off on the wrong path. Better give us all the details.
Leo..