My Apple ID region is Indonesia but I wanted to change it cause I can't download some app on the Indonesian store
I think this page can help you. But I'll explain it here too.
On your device, tap Settings > iTunes & App Store. Tap your Apple ID. If you're not signed in, sign in with the Apple ID and password that you use with the iTunes Store. Tap View Apple ID.
Your Apple ID country updates for all of your Apple services on all of your Apple products.
Open iTunes. If you’re not signed in, sign in with your Apple ID. From the menu bar at the top of your computer screen or at the top of the iTunes window, choose Account > View My Account. If you’re using Windows and can’t see the menu bar, learn how to turn it on. Enter your password, then press the Return or Enter key on your keyboard, or click View Account.
On the Account Information page: