To extend subscription - Sign In to VPN Shield with your credentials -> click on 'Extend the subscription' button -> you will get an options for subscription. Price depends on a country, but in general it is:
1 week - 1.99 USD
1 month - 4.99 USD
1 year - 29.99 USD
3 year - 69.99 USD
Select prefered subscription plan and click on 'Buy Subscription' button. VPN Shield will connect to Apple Store to proceed purchase.