If the bill is in Draft status, you can change the payee by selecting an alternative from the drop down list.

If you’ve already started setting up the payment, then click ‘cancel’ in the payment screen, which will take you back to ‘Bills to pay’. Click on the bill (in the Draft list), select a different payee and continue as before.

If the bill payment has already been scheduled, the only way to change the payee is to cancel the standing order and start from the beginning - upload the bill afresh.