Firstly, the sales order must not be marked as "Completed", once that is confirmed, then the process is as follows:
In Supply Chain, select Sales
Select Allocations from the Process menu
Select Change Allocs for Order
Enter the order number (or browse for it) and select Save
Select the line item you wish to change and click on Change This Allocation
Enter the required number and hit Tab key
On the Allocate Lots page, select the Lot to be allocated to the order
Confirm the quantity and click Save
If more than one lot is required to be changed, repeat steps 7 & 8 as required
Once all required lots have been updated and saved, close all windows