In some scenarios, it may be easier to just remove the field and create a new one. By doing this you will get help from the wizard. The wizard will walk you through which field this combo box will affect in the DB, what table to use as the list of options as well as what data to display or use.
You can also change the column the combo box uses to populate the DB by selecting the combo box in design view, clicking on the data tab on the property sheet and changing the 'Bound Column' to whichever column you want to use (1, 2, 3 etc.)
Hope this helps!
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