How can I change my region at the payment method?

You are trying to change your payment method after you changed regions?

Log into your account in the App Store/iTunes and you should be able to change/verify it there. Or you can click the blue Edit payment methods in the article below. When signed in and the page loads, you will be on the credit card page. After updating that, click the Go Back button at the lower left and you will be on the address page.

Apple ID payment information - change.  

To change regions, you must have a zero balance account.

From the article below.

• Spend any store credit remaining on your Apple ID. Learn what to do if your remaining store credit is less than the cost of a single item.

Region - Change your iTunes Store, iBooks Store, or App Store country or region.     

You are welcome.