I try to change refactor and change default namespace:
Hightlight namespace in MainPage.xaml.cs -> Right click -> Refactor -> Rename
Enter new namespace name and apply for all finded. That change Default namespace in project preferences too.
But after that my project doesn’t starts.
How do I change my namespace correctly?
You need to set the Startup Object, as detailed in the following blog post :-
http://nicksnettravels.builttoroam.com/post/2010/08/04/Windows-Phone-7-Beta-My-Application-Wone28099t-Start-After-Renaming-Project.aspx