Upgrade wp 2.9.2 multisite to wp 3.3 multisite


What is the best way to upgrade a WP 2.9.2 multisite to WP 3.3 multisite?

I have a wp 2.9.2 multisite with about 35 blogs/websites that i have to upgrade to latest version (wp3.3)

I also have some custom code i have du rewrite for wp 3.3.

Thanks in advance!