Please I want to change my payment method on my iPhone as my other card is expired

You can have a credit card / debit card on file and an Apple Account balance.

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

See the following Apple articles:

If you need to change or update your Apple ID payment method

If you need to change or update your Apple ID payment method - Apple Support

Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.