If the expression is the results of a calculation you do not want to save it in the table. Leave it as it is. Depending what you are doing you can do the calculation someplace else (maybe the After Update event of a control) and use code similar to this to add the results of the calculation to the field in question:
Me.FieldToSaveDataIn = YourCalculationHere
The FieldToSaveDataIn must be bound to a field in your table.
hth,
Jack