You could use a series if IF statements to check if the control has data and cancel the update if the control was Null amd/or ask if the record should be canceled.
(use your control names)
Code:
Sub Form_BeforeUpdate(Cancel as Integer)
'check Last Name control
If Len(Trim(Me.LName))=0 Then
Msgbox "Please enter the Last name", vbOKOnly, "Missing Last Name"
Me.LName.SetFocus
Cancel = True
End If
'check first Name control
If Len(Trim(Me.FName))=0 Then
Msgbox "Please enter the First name", vbOKOnly, "Missing First Name"
Me.FName.SetFocus
Cancel = True
End If
etc..
End Sub