Upgrading from version 3.7.7

Hi, I’ve just taken over managing a site that hasn’t been upgraded for years any tips on how to upgrade from 3.7.7 to the current version?

I’ve read http://docs.wpecommerce.org/upgrading-from-3-7-to-3-8/ do I do the same in steps to 3.9 and then 3.10 ?

My site is multilanguage and I’ll need to update the theme too, any recommendations for a theme?

Thanks

https://wordpress.org/plugins/wp-e-commerce/