This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
So: you read the file into memory. That's step 1. Then you made some changes to it in memory. That's step 2. But you're missing step 3, which is to write the workbook out to a file. Looking at the API documentation I can see a method that does that.