Default BACS Export Format is changed by following the below steps:
Select Financials, Cashbook (ribbon menu) or Cash Book, Cash Book Overview (module/classic menu)
Select the relevant bank account via the magnifying glass next to Code
Click on Bank Details
Select the Account Details tab
Select Edit
Select the BACS Export format button, followed by the format you wish to use.
Note that if you have multiple bank accounts you will need to do this for each of the bank accounts you need to change the format for.
You will now see the new BACS format on the Remittance BACS and BACS Export screens.