If you already have topped up, and so you have enough credit on your account, you can use it to buy membership either online on the 48 website (just choose to pay from your balance when it is asking for payment methods, instead of your card) or you can also buy via text message.
Check our guide here for how to buy stuff by text. Remember that if you still have active membership until 18th August, your new membership will only kick in once the active one has expired.
Can you let us know if that helped or if you're still stuck? Cheers!
