Payment
How to add Bangladesh as payment method?Bangladesh is not listed on payment options.Thats why I can't purchased or updated my device.
You can't, not until if/when Apple open an iTunes Store there (we are fellow users, we won't know if/when that might happen) - until it does, you won't be able to have an iTunes account and download any iTunes content.
That shouldn't affect the ability to update the phone's iOS version nor use it as a phone