I need to update the text on an AP Document, however the invoice document has been posted and I can't change the text field, how do I update?

In order to update the text information, find the relevant FI Document number. 
Open the FI document using FB02 (edit document). 
Double click on the line related to the vendor

Click on the button for the long text

Select the icon for editor

Enter the updated text in the field

Click the back arrow, then select copy text

Then save

The text will now be updated when you re-run ZAP_FIAUTH

Note: Umoja will always first check the FI Document for text. If there is no text, it will then check the invoice document.