Word itself can't flip text horizontally, and it can only work that
trick on some graphics.
However, your printer's driver software may be able to do it. In the
File > Print dialog, click the Properties button to open the printer
driver's dialog. Look through there (or the manual that came with the
printer, if you can find it) for an option that mentions transfers or
T-shirts or "flip horizontal". If it's there, it could be a check box
or a selection in a dropdown.