You need to log in to your WordPress admin with your username/email and password. Once you’re in the wp-admin dashboard you’ll need to navigate to “Pages > All Pages” (The menu names will be in French so navigate to the equivalent page). Under “All Pages” select the page you want to edit. You may also edit existing posts under “Posts > All Posts”
Thank you for your response.
I have some questions left.
In the panel you are reffering to I can’t find the Welcome page.
I am not able to remove the Modifier button. Removing the button is necessary because it’s assumed to become an WordPress Demo. For doing so I need to edit the page code.
I don’t understand your question. Can you upload a link to a screenshot of the page you want to edit and an explanation?
You can use a service like imgur.com to upload the screenshot.