To show that and M are isomorphic we have to define a bijection that satisfies all the three conditions given in the definition of DFA isomorphism.
Recall that the states of are where are the representatives of each k equivalence classes of .
Let us define as follows
That is, f maps state of to the state in M which can be arrived at processing the string from the start state of M. we know that . Hence f is well-defined.