If it relates to a current period transaction, then adjust the particular current period transaction.
If it relates to a prior period transaction, then create a Journal Entry.
Lets say, the original item was 110, 100 business expense and 10 GST. If that item is now only 50/50 business/private expense then you would have 110, business expense 50, private expense 55 and 5 GST.
So to make this adjustment you would do the following:
If it is a change of creditable purpose for property assets, it may require a series of adjustments. As a solution, you can create a Custom Tax Code named - GST Inc Adj. Create a journal entry debiting the cost amount that has changed the creditable purpose, without GST, and credit the cost allocating the credit amount to the Custom Tax Code.
Custom Tax Codes are not linked to the GST Calculation Worksheet but are seen in the Tax Summary.