To edit an existing page on your website, you must login to your website dashboard.

Go to: https://YOURDOMAIN/wp-admin
Replace "YOURDOMAIN" with your actual domain name.

After login to website dashboard,
Select  ---- PAGES   ---- ALL PAGES
 

Then Select the page you want to edit and click the page title to open edit mode.

This will allow you to edit any existing page on your website.