How do you change the year setting on Lego Harry Potter Years 5-7 once you've completed the main story?

To change the year setting in LEGO Harry Potter Years 5-7 after completing the main story, go to the Leaky Cauldron and enter the room where you can access the different years. Use the book on the table to select the specific year you want to play. This will allow you to revisit levels and collect any missed items or complete challenges in that particular year.