These are two separate verbs and must not be interchanged.
The principal parts of flee are flee, fled, fled;
those of fly are fly, flew, flown. To flee
is generally used in the meaning of getting out of danger. To fly means
to soar as a bird. To say of a man "He has flown from the place" is
wrong; it should be "He has fled from the place." We can say with
propriety that "A bird has flown from the place."