There are two ways of upgrading to the Premium version.
It’s possible to upgrade using the Pricing page of the WP Mobile Menu site or it’s also possible to upgrade in the account section of the WP Mobile Menu in the WordPress Admin area.
Using the Pricing Page
Go to the Pricing Page and select the desired version, license Number. You can change that later in the Payment Modal.
Using the WordPress Admin Area
Go to the Mobile Menu -> Account or Mobile Menu -> Upgrade and select Page and select the desired version, license Number.