YANK to POOP.

Every letter change

1. YANK → BANK: change letter 1 from Y to B.
2. BANK → BONK: change letter 2 from A to O.
3. BONK → BOOK: change letter 3 from N to O.
4. BOOK → BOOP: change letter 4 from K to P.
5. BOOP → POOP: change letter 1 from B to P.