I'm assuming you want to update Table2 when you click on the button. You'll need to write some VBA code for the On Click event of the button.
Basically, loop through the selected items in the ListBox and update using value from the combo box.
Here's a skeleton code to get you started.
Code:
Dim oItem As Variant
If Me!mylistbox.ItemsSelected.Count <> 0 Then
For Each oItem In Me!mylistbox.ItemsSelected
' Use DAO recordset to update those selected records
' either with SQL or using RecordSet.Edit and RecordSet.Update methods
' .....
Next oItem
Else
MsgBox "Nothing was selected from the list", vbInformation
Exit Sub 'Nothing was selected
End If