Upgrade from b2 to WP

I know this is a common subject ! :o) I read many topics about it, but I’m not sure I understood well what to do with database prefixes.
These are the steps I read on the install doc :
You must configure wp-config.php as indicated in the “5-minute install” section.
–> But for the // Change the prefix if you want to have multiple blogs in a single database.
$table_prefix = ‘wp_’; // example: ‘wp_’ or ‘b2’ or ‘mylogin_’
Do I have to put “b2” as I ran a b2 blog, and that my mysql table has b2 prefixe ?? (but I don’t want to have multiple blog… I just want to upgrade !!!). Therefore, b2 or wp_ ?? ??
All you really have to do is replace all the files with newer versions and run wp-admin/upgrade.php and you should be ready to go.
—-> Replace all the files with the same name from my former b2 directories with the WP ones ? Or delete all contents in b2 directories, and copy WP files ? Because in my b2 blog, files are not beginning with “wp_”, that is the case for WP !
There is also an import script at wp-admin/import-b2.php.
—> Both methods must be used, or it should work with the first one ? If we use only this one, what should be performed first ???
Well, I realize I may not be proficient enough to make the upgrade. In fact, I’m afraid to loose my preciouuuuuuus blog. Arf arf. ;))
Can anyone help me ? Anyway thank you very much (and sorry for the crap english).