If agitator shaft is chipped off at some portion then how to repair it ?

The solution to this problem definitely depends on the size and location of the chip, plus the condition of the overall shaft.

If the chip is just a surface chip (not all the way down to steel), you may be able to just cover it with a Teflon sleeve depending on its location. You may be able to locally repair the chip (essentially reglass just a small area) if it is in an easily accessible place and the shaft is relatively new. You may be able to reglass the entire shaft if the damage did not go all the way through the steel. Or, you may need to purchase a new agitator if the damage went through the underlying steel or if the chip is in a critical location.