To update the credentials for a bank account:
Go to the Manage Accounts screen (cog icon on the left hand My Accounts column)
Locate the account you wish to update and click/tap the three dot icon on the far right of the account, then select 'Update login details' to be taken to the credentials form for that provider.
Updating the login details on that particular account will also automatically update the login details for all other bank accounts/credit cards from that provider that share the same login details.
Please note it is also not possible to Update/Refresh an account within 30 minutes of a successful add/refresh of that account.