This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Replace all the files that come in the /lib directory when you download Struts. Then change the DTD that is referenced in your struts-config.xml file. If you're using the validation framework, you may need to change the referenced DTD in the xml files required by validation as well.
You shouldn't have to modify anything in your code. Some things are deprecated, but they are still supported.