if you take a look at the update page, there is a recordset that is filtered using a URL variable, this recordset is used to populate the form with the values to be updated.
to be able to insert and update on the same page, you will need to have 2 buttons for the form. In the insert behavior, set the trigger to Button insert pressed. in the update behavior, set the trigger to button Update pressed.
be aware, though, that using the form for double duty like can cause confusion. The user may go to update a record and click the Insert button instead causing the updates to be lost for the original record, and stored as a new record instead.
From a user perspective, it may not be a great idea to make a page that can do both, just because it can be done, does not mean it should.