I have hot water tank 220 v and I like to convert it to 110 v what I have to do ?

Buy a new water tank.

If you are using a 240 V tank, with dual elements, then they both require 240 Volts to heat the elements. Even though only one element at a time is on.

A 120 V feed, will not be able to sustain the current needed to properly heat the water in the tank.