Can't change text format

1

I have a textField inside a mc. When you roll over an event is triggered.

In the fuction I can do this

event.currentTarget.getChildAt(0).text="fred";

and the text changes - so its being targetted

I want to change the format of the text so I thought

event.currentTarget.getChildAt(0).defaultTextFormat=mTitle_fmt;

but, its doesn’t work.

it says “ReferenceError: Error #1037: Cannot assign to a method setTextFormat on flash.text.TextField.
at index001_fla::MainTimeline/overHandler()”

Any pointers please

thanks