If you want to upgrade your account, tap on the 'profile' icon on the toolbar at the bottom of the screen. You will see your profile picture. Just under your picture you will see an 'upgrade' button. Tap on the upgrade button and you will be shown all of the subscription options that are available for your nickname.
Scroll through the selections and tap on the 'subscribe' button for the type of subscription that you want to buy..
You will see a confirmation screen. Tap on the 'subscribe' button to accept and pay directly through the app store. Or tap on 'back' to go back and continue browsing the options.
You will receive a confirmation email. Be sure to save it for your records.