How can changes be in AC voltage to DC voltage ?

The simplest one is using diode bridge rectifier (4 diodes arranged in series pair). You can also use transformer with center tap combined with 2 diodes. However, to get smooth DC, you also need smoothing capacitor.

AC VOLTAGES ARE HIGH VOLTAGES USED FOR TRANSMISSION AND DISTRIBUTION PURPOSES.ie 11KV,33KV,132 KV etc

DC VOLTAGES LOW USED FOR ELECTRONICS POWER SUPPLY ie 5V,12V,24V etc