Go to "Accounts" section

Choose "Top-up"

Select amount you wish to to top-up

Select account you wish to top-up

If you have a card saved for top-up, use it, or just click "Continue"

You will be given an option to enter card details you want to make a top-up from or Apple Pay

Go to Mobile App

Select "Add money"

Choose the way you want to add money to your account