How to change region???

You cannot.

You can only use payment methods from the region your Apple ID is set to.

If you need to use a Bangladeshi payment method, then you need to change the region to Bangladesh.

See here: link-> Change your Apple ID country or region - Apple Support

Click on the link and read how to "Change your Apple ID country or region" - https://support.apple.com/HT201389

I think there are no services available in Bangladesh. You must use a payment method that is valid in the country to which your store is set which is also supposed to be the country where you reside.

Availability of Apple Media Services - https://support.apple.com/HT204411 - "Learn if you can buy apps, movies, music, books, and more, or use Apple Podcasts or Apple News, in your country or region." - If your country is not listed then you cannot buy anything (and there is no need for a payment method).

Actually it isn't in that link, it's in Payment methods that you can use with your Apple ID - Apple Support

Interestingly the full statement is:

"Make sure that the billing address for your payment method matches the billing address for your Apple ID. In some cases, you might need to call your bank or financial institution to authorize international or online purchases. "

Makes it seems you could have a "valid" payment method based in another country as long as you could make the billing address thing work.

You can only use payment methods from the region your Apple ID is set to.

That (along with many other requirements for store usage) is often stated but oddly I cannot find anywhere in Apple documentation where they say that. They just say it has to be "valid". My credit card is valid in Japan if I go shopping there but I suspect it might not be if I tried to use it in the iTunes Store there. Still, I avoid trying to say Apple says something when they don't actually.

From the link I posted: Make sure that the billing address for your payment method matches the billing address for your Apple ID

The Billing address set for your Apple ID is defined by the Country or region set for the Apple ID. ie. the address options are specific to the selected country, and things like postal codes are verified to match the selected country's format. This prevents usage of a billing address that does not match the address formats of the selected country.

You could possibly fake an address to match the format, but I believe the verification process matches at least some of the address information in some form to the payment method, so not sure fake addresses work.

And in my personal experience adding payment methods from a country different to the one set for my Apple ID has not been possible.