How can I change all the two prong receptacles without a ground mounted in plastic outlets boxes to 3 prong receptacles with ground ?

If your electricity is working properly, you are grounded.

If you do not have a ground, the best way would be to rewire the entire circuit from the breaker to the end of the circuit.

As an alternate, you can use GFCI receptacles and mark them as ungrounded.

Question, why are you so sure the circuit is ungrounded?