It seems you have set your table such that not accept null values. The easy way out is to remove this requirement. If not possible then you need to validate all your fieds on in the form's in BeforeUpdate event using:
If Nz(Me!txFName,"")="" Then
MsgBox "You need to enter a valid value for First Name."
Cancel=True
End IF
Add a similar code for each txt box with entery required setting in the table.
Mike