Minecraft wiki entry for Zombie_siege_defense, associated text: Splash them with a Potion of Weakness
Feed them a golden apple
Wait. The cure takes several minutes, so the player must prevent them from burning or otherwise dying (or despawning) before they recover. Luring them inside an empty house and then barricading them in can work well. If the zombie is damaged, a Splash Potion of Harming can be used to heal it a little, increasing its chances of surviving.