One of the best learning resources I have found since I joined my team at Microsoft is this page Danny Simmons has just published. Look here for his post explaining it. If you have any further questions that are not covered or you feel are not clear enough, feel free to add your questions in comments here or even email me.
Danny already answers lots of questions everyday in the forums, and I like "learning by explaining", so give me a chance :)
In VS 2008, after modify the database tables, we want to regenerate the same entity data model (same name) with modified database table, how do we do that?
Please try the "Update from Database" option by right-clicking on the design surface of the EDM designer.
Hope this helps,