Payment method is not changeable after the sign-up completion.
Please cancel/terminate the current contract and then make a new contract again.
You can check how to cancel/terminate here.
https://kdlsupporten.zendesk.com/hc/en-jp/articles/35656009217945
You can check how to make a contract here.
https://kdlsupporten.zendesk.com/hc/en-jp/articles/35655363532953