Hi,
Thanks for reaching out to OkadaBooks. To refill your account using the iOS app. Kindly login to your account. 
You will see the user icon before the four dots at the bottom right? Yes, that is it!
Click on it
You'll be taken to your profile page
There, you'll find your balance and a green button that says REFILL
Click on it to refill your account.
Note: If you have issues refilling with your card on iOS then see all our other refill options here
Regards,