This one's easy.

On Web:

1. Login and go to the 'blue person icon' at the top right

2. Go to 'My Account'

3. Select 'Payments'

4. Select 'Add a new card'

5. Pop in the details - make sure to tick to 'make this card default' if you are switching to that for all payments

6. Click 'save'

On App:

1. Go to 'Account'

2. Select 'Payment Methods'

3. Select 'Add a new card'. You also have the option to select Google Pay and Apple Pay here if you wish

4. Pop in the card details.

5. Once it has been added to set as default, select 'Payment Methods' and then the card with the blue square and tick is your default, to switch over, select the new one and click 'set up.'

You can also remove any cards you no longer require.