Suggest on modify C#.net 2008 app

Question

Would you advise me on what would be the best way to modify tables behind an existing C#.net 2008 non web application:

1. Would you  use refactoring? If so,what methods would you use ?
2. Would you modify an existing D/T diagram using a D/T designer? If so, do you have directions on how to use a D/T designer?

For my database change, the follwoing will occur:

1. Some tables will be replaced by new tables,
2. some columns will be removed from existing tables,
3. some columns will be added to existing tables,
4. the order of columns will probably change in  some tables,
5. the size of columns will change.