So in other words, the answer to the question posed is you CANNOT change the default file location.
There are of course ways to do a save-as or a file move, but none of those approaches changes the default file location as requested. TurboTax will create a TurboTax sub-directory under the logged-in user ID every time TutboTax is launched, and unless steps are taken to override the location (i.e., doing a save-as), will save to its default location.