Renewing is no different from paying the first time: press "Renew" in the subscription block in your account, or open the payment page and pay for the term you want. Paid days are added to the current end date, so you can renew early without losing what is left.
We do not store payment details and we do not take money automatically. The subscription simply ends on its date until you renew it yourself. The bot reminds you before the term runs out, so it is worth leaving notifications from @obfsnbot on.
The subscription link still exists, but working servers are no longer served through it: the app receives a limited service profile and a message saying access has ended. That is not a broken client; everything comes back once you pay.
The number of paid devices is chosen on the same payment page. If you set fewer devices on renewal than you have paid for now, the number of slots does not shrink: the largest number you have paid for stays. You can unlink extra devices and free slots on the devices page.