I would like upgrade my storage plan but I’m unable to do so

See if this helps: https://support.apple.com/en-us/HT203005

Glad_75 wrote:

It says my payment method was declined

A “payment declined” message means that the issuer of the credit or debit card you have on file in your iTunes account rejected a charge or account confirmation. It is not an Apple problem; Apple is just passing through the message from your issuer. There are a number of reasons the charge or verification might be declined:

1. Insufficient balance or account in arrears
2. Account suspended for suspected fraud (not necessarily by you)
3. Error in card number
4. Wrong expiration date
5. Wrong security (CVV) code
6. Address in your iTunes account does not match exactly the billing address for the card
7. The card was not issued in the country of the iTunes store
8. You are connected through a VPN provider that does not terminate in the country of the iTunes store you are connected to.

Are you getting any error message?

As soon as you tell what the error message said we will try to help you.

You can try to contact apple directly using the contact support link at the top of the page to see if they can help you.