When you order your SIM card for the first time, you have to choose between Flat 4, Flat 10 and Flat 300. In return, you get one month free to test our offer. 

Once you have activated your SIM card in our customer portal, you can make precise adjustments to your subscription. Then you also have the option to choose the Flat 50 or the Flat 1 subscription.

A subscription upgrade is always possible, we would charge you the difference of the two subscriptions pro rata. 

A downgrade is only possible after the expiration of your selected subscription, which runs for 30 days.  

You can find out how the whole thing works here:
How can I manage, deactivate or pause my subscription in the cockpit?