If you're trying to remove a residue, there are solvents for this stuff, but, alas, they only work before it's fully cured. After curing, the only way to remove it is by 'mechanical means', e.g. by scraping it off with the likes of a (very sharp) broad chisel or a Stanley knife blade. if you're careful, and don't gouge the uPVC, this should leave a fairly smooth surface that can be made to look good as new by energetic rubbing with a damp cloth and a cream cleanser like Jif (Cif).
But if you mean the uPVC has been discoloured by the foam, then there's no telling how deep the stain will be. In this case there's little you can do to diguise it except by either painting the door, or possibly by replacing just the affected piece. (See Yellow Pages under 'Double Glazing' for those who do uPVC repairs).