There are two ways of upgrading to the Premium version.
It’s possible to upgrade using the Pricing page of the Menu Image site or it’s also possible to upgrade in the account section of the Menu Image 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 Menu Image -> Account or Menu Image -> Upgrade and select Page and select the desired version, license Number.