How to change billing country in iPhone's Apple Account

before you can change your country and region you'll need to fulfill certain requirements:

• Spend any balance remaining on your Apple Account. If you’re not sure whether or not you have a balance, check your Apple Account balance.
• Cancel any subscriptions that block a country or region change, and wait until the end of the subscription period. 
• Wait for any memberships, pre-orders, movie rentals, or Season Passes to complete, and wait for any pending store credit refunds to process. You can cancel a pre-order.
• Make sure that you have a payment method for your new country or region. See what payment methods you can use.
• Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. Learn what types of content and services are available in your new country or region.
• After you update your Apple Account billing address and payment method, you can upgrade your iCloud+ plan using the pricing in your new country or region.

After this you can change your country or region:

1. Open the Settings app. 
2. Tap your name, then tap Media & Purchases.
3. Tap View Account. You might be asked to sign in.
4. Tap Country/Region.
5. Select your new country or region.
6. Tap Change Country or Region.
7. Tap your new country or region, then review the Terms & Conditions.
8. Tap Agree in the upper-right corner, then tap Agree again to confirm.
9. Select a payment method and enter your new payment information and billing address, then tap Next. You must enter a valid payment method for your new country or region.*

You change hte country setting on your acount. The article Change your Apple Account country or region - Apple Support describes how to change your Apple ID country or region, and the requirements and results of doing this.

You may encounter various issues and here is how to deal with them:

● Subscriptions

Make sure you have canceled all your subscriptions (see: If you want to cancel a subscription from Apple - Apple Support ) and wait for any memberships, pre-orders, movie rentals, or season passes to fully expire. Items will still be active until their expiration date. You need to contact Apple and request any ongoing items you have be ended immediately if you do not want to wait until they expire naturally.

● Remove all your Apple Account balance using one of these ways:

- Add a credit or debt card as a payment method to your Apple ID and buy items just in excess of your remaining Apple ID balance. This will use up all the balance and any excess will get charged to the card, leaving nothing in your account.

- Contact Apple using this link --> https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0998&symptom_id=20198 and ask them to remove your balance (they will keep it).

● From the first link above: "Make sure that you have a payment method for your new country or region." Refer to: Payment methods that you can use with your Apple Account - Apple Support Note that some in that list may not be acceptable to set as a payment method.; for example, account balance. The statement from Apple, and its referring to the link, to me just says that if you were using a payment method in your old country that is not listed in the new country too then you need a new payment method. I do not see anywhere where it says the payment method must be from the same country. I go by what Apple actually says in its online documentation. They appear to deliberately avoid making a simple, clear statement which suggests to me they have more complex criteria other than it must be a payment method from the same country. If it says a method is not valid then you need to try another one until you find one that is. Still, often using one from your new country is the easiest way to ensure you don’t run into problems.

● Pending purchases and refunds

If you have pending purchases, or refunds in the process of being returned to you, you will have to wait until these clear.

● Leave or disband any Family Sharing groups.

Refer to Leave Family Sharing - Apple Support

● Read this support article If your Apple ID or other options are greyed out in Settings —> If your Apple Account is grayed out in Settings - Apple Support

For items where you need to contact Apple to resolve an issue:

Click here ➞ Choose your country or region - Official Apple Support

Select your country (if you do not see your country, also look for "other" regions; e.g., "Other Africa"), then "Choose a product". Click the "show" item under each category for more selections. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

See a list of Apple phone numbers around the world. Click here --> Contact Apple for support and service - Apple Support