Go to 'Dashboard' - 'Banking'
Click on the 'Edit' icon (left next to the bank balance) on the bank account 'block' you want to edit.
You'll see the account name and opening balance. At the top is a link, 'More Bank Details...'
Click on this link and update your bank details.
Please note: If you have more than one bank account on BeanBalance, the details of the default account will be displayed on your invoices. There is a check-box on the 'More Bank Details' that you can click to make an account the default account.