A family of Microsoft relational database management systems designed for ease of use.
Create an After Update event procedure for the first dropdown. Here is the basic idea; you can adapt it for your situation.
Private Sub ComboBox1_AfterUpdate()
If Me.ComboBox1 = 3 Then
Me.ComboBox2 = 5
End If
End Sub
In this example, both combo boxes have a number field as bound column. If they have a text field as bound column:
Private Sub ComboBox1_AfterUpdate()
If Me.ComboBox1 = "Specific Value" Then
Me.ComboBox2 = "Another Value"
End If
End Sub