You are travelling on balance. Then NS does not withdraw money from your bank account, so there is no bank account number you can change.

Do you use automatic reloading? Then you can change the bank account number for that in Mijn OV-chip on www.ov-chipkaart.nl.

If this doesn’t answer your question, please be more specific about for which product you wish to change the bank account number.

Thank you, Tochjo. I substribed a NS flex, and I want to change the bank number which paid for the NS Flex. Do I need to do so in www.ov-chipkaart.nl?

No, then you do need to change the bank account number in Mijn NS. However, the image you attached does not show any NS Flex product.

Do you actually have NS Flex active on the card that is shown on the image? If so, you need to contact customer service directly to have them sort out what is wrong, because you should be able to see NS Flex on the page in the image, and you should be able to change your bank account number there. If NS Flex is not active yet, you need to activate it first.

To be clear, you are at the right page to change your bank account number; for me, it shows the following:
 

Contacting customer service can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Note that it can take a few days to receive a reply if you contact them online.

Yes. I’m sure I had a NS Flex. I just recived my card today by purchasing a NS Flex online. Do I need to wait for several days or 24 hours probably?

When you ordered NS Flex, you had to enter a starting date. I'm guessing that date hasn’t passed yet? Then you need to wait until that date. You will not be able to see your NS Flex product in Mijn NS before the starting date.

OH, that’s it. My start date is 27 Oct. I should change my bank account tomorrow. Thanks a lot!