If your fender is rubbing against the tire, you want to first make sure that it was installed properly as per the manual included with the bicycle. The metal support struts that help support the fender can be bent and adjusted manually, by hand, to help center the fender.
With a firm grip near the attachment point where this strut meets the bicycle, bent the strut away from the tire itself on the side which it is rubbing. Manual adjustment of these struts on both sides will resolve the issue. Don't be shy! You can bend the metal supports without damaging them.