If the Beneficiary has been fully authorised, you will need to delete this Beneficiary and set up a new Beneficiary with the new account details.
If the Beneficiary is awaiting authorisation you can select the Beneficiary you wish to amend and click 'Edit'
Go to the Payments option on the left hand side menu / Payments Management / Beneficiaries.
Click on the Beneficiary you wish to amend and click either 'Delete' or 'Edit' (see note above).
Make sure you have the Sorting Code and Account Number for the beneficiary account.