To upgrade to a premium account click on the store on your home tab as shown below.
You will then be directed to the Palringo store where you can fins Premium account at the bottom left of the screen select it.
Once you have selected the premium section you will be directed to a confirmation page click select.
Once you have clicked select it you will be asked who you wish to purchase the premium account for select which user you wish to give it to and how long for.
You will then be asked to confirm your purchase.