I can't seem to update my payment method for my apple account - it has told me to contact support

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

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

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

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK) 

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

Hello! I would as well suggest contacting Apple Support as they can provide more accurate assistance: Contact Apple for support and service - Apple Support

Have a great day!