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