That can be easily done from the app.
Go to your Settings section first. You will see this view:
Click on SIM CARD and it will show you an option to Top up your SIM card:
Click on 'TOP UP SIM CARD' and you will be sent to a page where you will be able to pay for you SIM card.
In general - you will be charged 3 EUR/ 3.5 USD/ 2.7 GBP monthly fee (paid for the full year in advance, as a 36 EUR/ 42 USD/ 32.5 GBP annual fee). You will receive two notifications before the end of the free GSM service period (which is 2 months).