The short answer is: one of the polarization components need to interfere destructively.
One can always express any state of polarization as a superposition of two mutually orthogonal states of polarization, such a horizontal and vertical linear states of polarization or left- and right circular polarization. In this way the polarization is represented in terms of two components.
If you prepare the states of polarization of two beams so that one of these components are in phase ebtween the two beams, while the other is out of phase, then, upon combining the two beams, the components that are in phase will interfere constructively and the component that is out of phase will interfere destructively. The result would be that only the component of the state of polarization that interfered constructively would survive. In this way interference changed the state of polarization.