I keep getting a message to update my Apple Pay details

Change / update your billing address:

(macOS) Set up your Apple ID payment method and shipping address on Mac

Set up your Apple ID payment method and shipping address on Mac – Apple Support (UK)

or 

Sign in to your Apple ID account page and edit / update your Apple ID details -> https://appleid.apple.com

• edit your billing addres.

or 

To change the billing address you can do (macOS & Windows OS) via iTunes

Start Music (-> Click on iTunes) -> side bar click iTunes Store -> go to Account (top bar) -> View Account -> Sign in -> Apple ID Summary -> Billing Address -> Edit.

or 

(iPhone & iPad) Go to -> Settings -> tap your Profile -> Payment and shipping -> Change Address.

Edit the Address -> tap Done.