The Ritual of Chüd
After receiving his vision of It's ancient origins, Mike then stole the 18th century artifact that the Native Elders told them was needed to perform the Ritual of Chüd, which is how Mike believes the Losers will be able to defeat It. Although the Ritual of Chüd is indeed a big part of the book and vital to the Losers' success in both timelines, the mechanics of it are totally changed for the film. In the book, Chüd is a battle of wills, pitting the imagination of children against the ancient evil of IT. In the film, it's a ceremony, complete with props, which Mike believes must be followed to the letter in order for the Losers to be victorious.
After the Ritual of Chüd fails in It Chapter Two, Pennywise tells them that it didn't work because it was a "gazebo," implying that it was a placebo and that it had already failed once before. To the shock of the Losers, Mike then admits that he knew that the Ritual of Chüd failed the last time it was used but that he hadn't told his friends because he was sure that if they truly believed it would work, then it would.
This is a major departure from the book, in which the Ritual of Chüd not only works (albeit in a completely different way) but was never used against IT before and has nothing to do with the hospitalized Mike Hanlon, who is only ever honest with his friends.