在java中将文件重命名为另一个文件

I have a file I need to rename to that of an existing file. This is a copy, modify, replace original operation on an existing JAR file. I've got the first two steps done, I just need help with the replace original bit. What's the best way to rename the new version of the JAR to that of the old. The old JAR doesn't need preserving and I don't want to have a copy of the new with its initial name sticking around. I have a file I need to rename to that of an ex