Your question is confusing. You talk about a checkbox, so that means user interface. So far so good, but why modify the checks if the user is editing it ?
Unless you mean modifying the choices made in a document that is saved, in that case you're just modifying a field value.
Or, you want to modify the values in a checkbox displayed on a browser.
So could you please specify ? Is the document open or not ? Is it via browser or Notes client ? Is the agent calling the checkbox or modifying a checkbox made by another process ?