how to edit the bitrix page if there is no control panel?

This question is not possible to give a definite answer. It all depends on exactly what page you want to edit. For a full answer, you better study the official bitrix courses. Content Manager and Basic Administrator.

If you just "lost" the control panel: restore it. To do this, after the body tag (most likely in the header.php of the site template) you need to insert the code

<div id="panel"> <?$APPLICATION->ShowPanel();?> </div> 

You can also do this in the editor in the admin panel: Admin - Settings - Product Settings - Sites - Website Templates - Your editing template and there the first tab "Template" has a text field containing header.php and footer.php together.