The All’s Well That Ends Bell side quest in Hogwarts Legacy can be obtained during the Stop Ranrok and Rookwood main objective after receiving the Headmistress Speaks main quest. Travel to the Defense Against the Dark Arts Tower Floo Flame in the Astronomy Wing then head down the nearby flight of stairs to the lower level. Speak with Evangeline Bardsley near the railing who is complaining about the bells being taken down.
Travel to the Bell Tower Courtyard Floo Flame in the Bell Tower Wing, then head up to the Music Room (at the top of the stairwell through the southwestern door). Once you enter the room, take the stairs up to the bell town. You’ll need to use Wingardium Leviosa to put the two bells into place. You can find them using Revelio. After you’re done with that, return to Evangeline to complete the quest. You’ll receive the Erumpent Horn conjuration as a reward.