Step 2 · Apply Penetrating Oil
Since the flange is stuck, hardened plumber's putty and corrosion are likely culprits. Apply a generous amount of penetrating oil, such as PB B'laster or WD-40 Specialist Penetrant. Spray the oil directly onto the seam where the top edge of the flange meets the bathtub, allowing it to seep down into the threads. Also, try to spray some directly onto the threads from inside the drain. Let the penetrating oil sit for at least 30 minutes; for a very stubborn flange, allowing it to penetrate for several hours or even overnight can make a significant difference.