Hi,
you can requery a combobox using the name of the combo box and "Requery", like this:
Me.NameOfCombobox.Requery
If the combobox is inside a subform you need to add the path to it:
Me.NameOfSubformContainer.Form.NameOfCombobox.Requery
If it should be requeried when the record navigation is used you should add this inside the "Form Current" event in the main form which fires on every record change.
Cheers,
Christian