This can be changed in the Debtors Control:
Select the Control tab.
Select Debt Control.
Click on Bill Enquiry.
Put in the bill reference and select Search or enter.
Highlight the bill you require and click on Edit.
Go to the Debt Breakdown Tab.
Select the radio button Manual Allocation.
Select Search/New if you want to add a new contact if you want to just apportion differently just amend the figures in the grid.
Select the new contact if you are adding a new one and amend the allocation.
Then Update at the bottom.