Tables :: How To Uncheck A Checkbox In Form

I have a table "LightDuty" with a field [Email Sent] that is a YES/NO field.

I do not have a checkbox on the main form for the [Email Sent]; instead this field is checked (in the table) when the user sends an email to the receipient. I use the [Email Sent] field as a criteria for my query.

I need to uncheck the checkbox when the user changes the date in a field called [Drs Excuse Exp].

This is what I have tried (in the AfterUpdate event of the Drs Excuse Exp control):

Code:
Dim db as DAO.Database
Dim rs as DAO.Recordset
Set db = CurrentDb
Set rs = db.OpenRecordset("LightDuty")

[Code] .....

I get the runtime error 3265 "Item not found in collection"