Is there any way to do this with an Access Update Query.  I get the message "....update on linked table "Item" failed.  "...Not Supported"(#10003)

I tried also in single user mode...same message. 

I lost my data for these fields when I imported an *.iif file through through QB import, export functions. QB support finally admitted they had the same problem when trying to update the assembly items; their custom data also disappeared.

Not a problem, in this example I will try to change the CustomFieldColor for a Assembly header. First I need to determine the exact ItemInventoryAssemblyLine row I would like to update:

Select CustomFieldColor, ListID, ItemInventoryAssemblyLnSeqNO,
ItemInventoryAssemblyLnItemInventoryRefListID
from ItemInventoryAssemblyLine
where ListID='430001-1071511103' and ItemInventoryAssemblyLnSeqNO = 1
and ItemInventoryAssemblyLnItemInventoryRefListID='130000-933272656'

I then update the CustomField with the color value I want, for example 'Red':

UPDATE ItemInventoryAssemblyLine SET CustomFieldColor = 'Red'
where ListID='430001-1071511103' and ItemInventoryAssemblyLnSeqNO = 1
and ItemInventoryAssemblyLnItemInventoryRefListID='130000-933272656'

And all CustomField values were updated for the assembly (header custom field):

Select CustomFieldColor, ListID, ItemInventoryAssemblyLnSeqNO,
ItemInventoryAssemblyLnItemInventoryRefListID
from ItemInventoryAssemblyLine