To populate check boxes on your update page do the following:-
On the update, page, you set up the checkboxes the same way as on the insert page.
Create a recordset for your chosen table (the one that holds the data for the checkboxes) to return the chosen ID and name, and use a repeat region to write a checkbox for each available checkbox echoing the ID as the value and the name as the label.
Once you add the Manage relational table behavior, it will pre-populate the checkboxes based on the selections in the checkbox table.
Hope that this helps