To upgrade an account to premium You will need to open up the store from your dashboard.
Then scroll to the bottom where you will see an option to buy a premium account with your Palringo credits. As shown below.
Click on the premium account option then you will be directed to a page that looks somthing like this.
From here you choose your contact choice, choose either yourself or a contact to purchase it for, so you can buy premium for yourself or for a friend.