Thats a pretty generic question. Have you tried googling for migrating from 2.x to 3.x? There are lots of books and online articles on it.
Off the top of my mind, I enjoyed reading EJB3 in action and that has a chapter for migrating.
It will read the ejb-jar.xml and actually insert the annotations in the code. Note this is maybe 1/3 of the effort. Converting Home and LocalHome to be business interfaces will take some work. As well if you have heavy use of CMP 2.x that will be quite a lot more work to convert to JPA.