ＪＰＢＡＮＫ登録された電話番号をどうやって変わりますか？How do I change my JP Bank registered Phone Number?

I recently got a JP Post Bank Account, but I now need to change my registered phone number to a different one, but I do not know how.
It doesn't seem like their website is capable of changing online, because i saw no such options on the main page....

How do I change my registered phone number that I gave the JP Bank?

or do I have to walk over to my branch bank and tell them directly to change my registered phone number???

Thanks, I figured it was gonna be like that (how can Japan keep living in the 18th century like that)